bluuebunny/crossref_metadata_2025 huggingface dataset:
Gabbroic xenoliths from the Northern Gorda Ridge: Implications for magma chamber processes under slow spreading centers
Authors: Davis Alicé S. , Clague David A.
Published: February 2008

Abundant gabbroic xenoliths in porphyritic pillow basalt were dredged form the northern Gorda Ridge. The host lava is a moderately fractionated, normal mid‐ocean ridge basalt with a heterogeneous glass rind (Mg numbers 56–60). Other lavas in the vicinity range from near primary (Mg number 69) to fractionated (Mg number 56). On the basis of textures and mineral compositions, the xenoliths are divided into five types: Type A xenoliths consist of large clinopyroxene oikocrysts, enclosing euhedral to subhedral plagioclase. Olivine and glass are present only in trace amounts. Type B xenoliths consist of olivine and plagioclase (±spinel) with abundant intersertal glass. Partially resorbed olivine with deeply embayed margins in places subophitically encloses small plagioclase crystals. Olivine and large plagioclase crystals often contain glass inclusions and/or spinel. Type C xenoliths are a composite of types A and B, with areas of large clinopyroxene oikocrysts adjoining areas of loosely joined clusters of olivine and plagioclase in glass. Type D is represented by a single large xenolith that, except for containing a larger proportion of crystals, appears identical to the host lava. Type E is represented by a single small xenolith that is mineralogically similar to type B but with a distinct, fine‐grained, tightly interlocking texture, formed by small olivine oikocrysts enclosing euhedral plagioclase, in a small amount of intersertal glass. Many single crystals and glomerocrysts in the host lava appear to be xenocrysts. Some forsteritic olivine and anorthitic plagioclase contain glass inclusions more primitive than the host lava, while rare glass inclusions in skeletal olivine suggest entrapment of melt more evolved than the host lava. The overabundance of plagioclase crystals and their textural features suggest accumulation. The xenoliths are not cognate to the host lava, but they are genetically related. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. Plagioclase‐rich leucogabbro layers in ophiolites attest to similar processes in magma chambers of past spreading centers. However, xenoliths in ferrobasalt from the southern Juan de Fuca Ridge differ significantly from the Gorda Ridge xenoliths and suggest that the magma chamber at the southern Juan de Fuca Ridge is located at shallower depth and in cooler crust than the one at the northern Gorda Ridge.
Gravity estimation from STAGE, a Satellite‐to‐Satellite Tracking Mission
Authors: Jekeli Christopher , Upadhyay Triveni N.
Published: February 2008

An analysis was performed to determine the accuracy of estimating gravity anomalies using satellite‐to‐satellite tracking between the Space Transportation System (NASA's space shuttle) and the constellation of GPS satellites (STAGE: Shuttle‐GPS Tracking for Anomalous Gravitation Estimation). Second time derivatives of the observed ranges, obtained from carrier phase measurements, between the low and high satellites provide in situ line‐of‐sight components of gravitation, as long as on‐board accelerometers are used to correct for the nongravitational signals. Absolute (nondifferential), single‐difference, and double‐difference modes of tracking the GPS satellites were considered in the analysis, which showed that the last yields the highest gravity estimation accuracy. With instrumentation already (or soon to be) available, it is anticipated that the gravitational acceleration at altitude can be measured to an accuracy of about 0.3 mGal (1 mGAl = 10−5m/s2≃ 1 μg) for an integration time of 60 s. Double‐difference acceleration data collected by the shuttle at 300 km altitude could, depending on mission duration and data processing techniques, yield estimates of 2°‐mean gravity anomalies on the Earth's surface to an accuracy of about 5 mGal. Accuracy in 1°‐mean anomalies of about 4–5 mGal would be achievable using a free‐flyer at 160 km altitude and assuming a potentially feasible measurement accuracy of 0.1 mGal (30 s integration time).
Correlation of paleotectonic fracture and microfracture orientations in cores with seismic anisotropy at Cajon Pass Drill Hole, southern California
Authors: Blenkinsop T. G.
Published: February 2008

Fractures and microfractures in oriented cores from the Cajon Pass drill hole, southern California, have strong preferred orientations. There are two major sets of extensional fractures and microfractures that have northwest and northeast strikes and vertical dips. Relative and absolute densities of both sets vary greatly within a single core. The regional tectonic history implies that these fractures and microfractures probably formed before late Miocene‐Pliocene. Vertical seismic profiling experiments in the drill hole show shear wave splitting with polarization directions parallel to the fracture and microfracture orientations. The measured seismic anisotropy is due to these inherited deformation fabrics and does not appear to be caused by the current local stress field.
Synthesis of body wave seismograms from point sources in anisotropic media
Authors: Tsvankin I. D. , Chesnokov E. M.
Published: February 2008

A method of synthetic seismogram calculation in arbitrary anisotropic media is developed. A solution in the frequency domain is derived in the form of a double integral with respect to horizontal slowness and azimuthal angle. Approximation of the influence of azimuthal anisotropy by Fourier series expansions makes it possible to carry out the integration over azimuthal angle analytically using series coefficients derived from simultaneous linear equations. As a result the double contour integral is reduced to a Fourier‐Bessel integral over horizontal slowness which retains information about azimuthal anisotropy. Numerical evaluation of this integral enables us to compute body wave seismograms with allowance for nongeometrical effects. This approach can be applied to reflection‐transmission problems or to the calculation of individual wavelet propagation in multilayered anisotropic media. The influence of anisotropy on point source radiation in a homogeneous medium is analyzed. Comparison of numerical results with high frequency asymptotic solution provides a reasonable physical interpretation for wave propagation anomalies. Synthetic data for a medium of orthorhombic symmetry show that the radiation pattern and the polarization of shear waves are very sensitive to the presence of anisotropy. The most pronounced amplitude distortions are caused by focusing of energy near velocity maxima and by defocusing near velocity minima. Numerical modeling reveals significant nongeometrical phenomena in anisotropic media. In some cases deviations from geometrical seismics are substantial even in the far field.
Analysis of mean sea level data from New Zealand for the period 1899–1988
Authors: Hannah J.
Published: February 2008

Records from four New Zealand tide gauges were analyzed in order to determine the trends in mean sea level which have occurred in New Zealand since the beginning of this century. Although a substantial difference in the computed sea level trends between tide gauges was considered a possibility, due to the location of the gauges in relation to the boundary of the Australian and Pacific tectonic plates, the analyses did not show this to be the case. They indicated rising trends in sea level of 1.3, 1.7, 2.3, and 1.4 mm/yr for Auckland, Wellington, Lyttelton, and Dunedin, respectively, and together gave a mean trend of 1.7 mm/yr for the east coast of New Zealand as a whole. These figures not only add to the global picture of sea level change but also provide initial evidence suggesting little or no recent differential vertical motion across this portion of the Australian and Pacific tectonic plates. In addition, they provide conclusive evidence that the orthometric height datums established in New Zealand from tide gauge data collected in the early part of this century are in need of substantial revision.
Source parameters of earthquakes and intraplate deformation beneath the Shillong Plateau and the Northern Indoburman Ranges
Authors: Chen Wang‐Ping , Molnar Peter
Published: February 2008

We determined the fault plane solutions and focal depths of 17 earthquakes beneath the Shillong Plateau and the northern Indoburman ranges by combining results from the inversion of long‐periodPandSHwaveforms and amplitudes, from polarities of first motions, and from the identification ofpPandsPphases on short‐period seismograms. Fault plane solutions of 15 earthquakes show mixtures of thrust and strike‐slip faulting, but thePaxes for these events are nearly horizontal and consistently oriented north‐northeast–south‐southwest. All of these earthquakes occurred at depths greater than 29 km. Beneath the Shillong Plateau, one event occurred at a depth of 52 km. The relatively large depths for earthquakes in an intraplate setting suggest that the Indian lithosphere in this area is especially cold. Earthquakes beneath the northern Indoburman ranges define a gently east‐southeast dipping zone from 30 to 45 km beneath the Bengal basin to 40 to 90 km beneath the ranges. This zone seems to steepen and connect with the zone of intermediate depth seismicity that dips eastward beneath Burma. These earthquakes cannot have occurred along the interface between a subducting Indian plate and the overriding Indoburman lithosphere, because thePaxes, not the nodal planes, are parallel to the north‐south trending seismic zone. Although a couple of the earthquakes might have occurred within the Indoburman lithosphere, most of this seismicity seems to have occurred within the Indian plate, recently and currently being subducted eastward beneath the Indoburman ranges. The consistent north‐northeast trend of thePaxes implies that the orientation of maximum compressional strain in the Indian plate throughout its northeastern pan is nearly perpendicular to that responsible for roughly north‐south trending folds of the Indoburman ranges. Thus, either recently in geologic time (since 1 Ma) the orientation of maximum compression changed dramatically, or, more likely, the deformation in the Indoburman ranges is decoupled from that in the underlying Indian plate. Meanwhile, the seemingly identical northward displacement of India and the Indoburman ranges with respect to south China must be accommodated farther east, along the Sagaing and other faults.
Speculations on the origin and evolution of the Utopia‐Elysium Lowlands of Mars
Authors: Frey Herbert V. , Schultz Richard A.
Published: February 2008

The origin of the overall low and variable topographic character of the northern plains is a fundamental aspect of the Martian crustal dichotomy. A model for the development of the Utopia‐Elysium lowlands in eastern Mars in terms of the long‐term evolution of large, overlapping impact basins is presented. The effects of relaxation of basin topography, volcanic flooding and loading of the basins with subsequent subsidence and downwarping, and the growth of a long‐lived major tectonovolcanic center in the overlap region between the proposed Utopia and Elysium impact basins are qualitatively described. In an early hot, thin (<50 km thick) lithosphere with low mantle viscosity (<1022 P), relaxation of topographic relief was probably rapid (<104 years) by comparison with the likely time between the two largest impacts (>106 years). The later Utopia Basin impact drastically altered the structure of the earlier Elysium Basin, uplifting portions of what had been the central depression. The heating and fracturing due to the impacts was most intense in the region of overlap, which probably provided an easy conduit for volcanic eruptions in this area. Loading by Early Hesperian and older basin volcanics caused widespread subsidence which may have extended beyond the basin rim and which was responsible for the generally low character of the northern plains, despite the minimum 2–3 km volcanic fill present there. Prolonged volcanism (through the Early Amazonian) would likely keep the overlap region hotter longer, opposing later general subsidence of the impact basin floor where it had cooled. Slower cooling and thickening of the lithosphere in the overlap region is consistent with lower estimates of the Early‐Middle Amazonian elastic lithosphere thickness (50 km) in central Elysium, by comparison with inferred greater thickness in the Early Hesperian near the edge of the Utopia Basin at Isidis (>120 km). Prolonged volcanic construction in the overlap region was also probably responsible for the great height of the Elysium tectonovolcanic complex by comparison with the other portions of the Utopia‐Elysium lowland, where most Volcanics are Hesperian in age. We suggest this model may be applicable to the origin of the northern lowlands elsewhere on Mars and that overlapping large impacts may be important in the establishment of the fundamental topographic and crustal dichtomy and the origin of major tectonovolcanic complexes on the planet.
Emplacement of lava flow fields: Application of terrestrial studies to Alba Patera, Mars
Authors: Lopes Rosaly M. C. , Kilburn Christopher R. J.
Published: February 2008

Morphological data are at present our major source of information for extraterrestrial lavas. Effusion conditions must therefore be inferred from the final shapes of flow fields, generally using terrestrial lavas as analogues and so presupposing similar emplacement regimes on Earth and other planets. Studies of terrestrial lavas suggest that the overall development of flow fields is systematic and that a general, normalized relation can be established linking the final dimensions of a flow field (specifically, average thickness and the ratio of maximum width to maximum length) to underlying slope and eruption duration, independent of explicit knowledge of discharge rate, gravitational acceleration, lava density, and rheology. This relation is applied to lavas on the Martian volcano Alba Patera, on which two distinct planimetric types of lava flow fields are identified, and eruption durations, average discharge rates, and average velocities are obtained. Imposing the constraint of a terrestrial emplacement regime, the model yields internally consistent results for subliquidus lavas and suggests that, at least for basaltic‐basaltic andesitic compositions, the essential conditions of eruption may have been similar to those currently observed on Earth.
Iron‐sulfur mineralogy of Mars: Magmatic evolution and chemical weathering products
Authors: Burns Roger G. , Fisher Duncan S.
Published: February 2008

Models for the evolution of sulfide minerals on Mars and reaction pathways to their oxidative weathering products in Martian regolith have been proposed based on petrogenetic associations between komatiitic rock types, Viking geochemical data, SNC meteorites, and terrestrial Fe‐Ni sulfide deposits. To test the weathering model, komatiitic pyrrhotites and olivines were exposed to sulfuric acid solutions, with and without dissolved ferric iron added to simulate deep‐weathering processes, and the reaction products were identified by Mossbauer spectroscopy. Secondary FeS2 (pyrite or marcasite), FeOOH (goethite), and possibly jarosite were formed from pyrrhotite, while olivine was oxidized to nanophase goethite. These results suggest that on Mars, acidic groundwater induced pyrrhotite → FeS2 → FeOOH (+ jarosite) oxidative weathering reactions, particularly in the presence of dissolved Fe3+. Such gossaniferous materials occurring in Martian regolith were derived mainly from Fe‐Ni sulfides associated with komatiitic basalts and not from sulfides occurring in calc‐alkaline porphyry copper deposits, granitic hydrothermal veins, sediment‐hosted PbS‐ZnS ores, etc., which presumably did not evolve on Mars due to the virtual lack of plate tectonic activity there.
Blue clearing of Syrtis Major at the 1982 opposition
Authors: Akabane Tokuhide , Iwasaki Kyosuke , Saito Yoshikazu , Narumi Yasunori
Published: February 2008

The degree of the blue clearing at Syrtis Major in the 1982 apparition shows dual components of variations. The phase‐dependent part increases as Mars approaches the opposition, and the diurnal component reaches a maximum near local noon at Syrtis Major. The phase‐dependent part is attributed to the opposition effect of the surrounding bright areas, and the diurnal variation is due to morning and evening clouds in the tropics. The opacity of the morning cloud is approximately unity in the early morning and decreases to null at about local noon.
Microearthquake evidence for extension across the Kane Transform Fault
Authors: Wilcock William S. D. , Purdy G. M. , Solomon Sean C.
Published: February 2008

We report the results of a microearthquake experiment conducted in 1987 in the active transform portion of the Kane Fracture Zone. The Kane is a slow slipping (25 mm/yr), large‐offset (150 km) transform delineated by a pronounced transform valley. The experiment site lies in the eastern half of the transform, 40 km west of the eastern ridge‐transform intersection, in a region of marked transform‐parallel topography. The network consisted of 12 ocean bottom hydrophones and seismometers and extended for 25 km along the transform and 10 km in width, spanning the transform valley and a narrow median ridge to the north. Hypocentral parameters were determined for 86 earthquakes. Epicenters within the network define a narrow transform‐parallel trend along the steep southern wall of the transform valley. Maximum focal depths increase from 6 to 9 km from east to west across the network. Significant activity was also detected outside the transform tectonized zone. To the southeast this activity may be associated with the inside corner of the ridge transform intersection, while to the north about 20 events occurred in 7‐m.y.‐old crust in a region of ridge‐parallel bathymetry. Six focal mechanisms were obtained from P wave first motions for events within the transform valley. The best constrained solutions, for four earthquakes within the network, show normal faulting on fault planes subparallel to the trend of the transform. All the mechanisms indicate a tension axis perpendicular to the trend of the transform. These results together with a significant historical record of large earthquakes near the experiment site lead us to conclude that the principal transform displacement zone was inactive during our experiment and that the activity we recorded is the result of extension in the adjacent lithosphere. The observed focal mechanisms and the inference that the axis of least compressive stress is approximately perpendicular to the transform provide direct evidence that the transform fault is mechanically weak relative to the surrounding lithosphere. Potential sources of extension across the transform include thermal stress in the young oceanic lithosphere, topographic loading, a small component of plate divergence normal to the transform, and northward motion of the asthenosphere relative to the surface plate.
Non‐Newtonian rheology of igneous melts at high stresses and strain rates: Experimental results for rhyolite, andesite, basalt, and nephelinite
Authors: Webb Sharon L. , Dingwell Donald B.
Published: February 2008

The stress‐strain rate relationships of four silicate melt compositions (high‐silica rhyolite, andesite, tholeiitic basalt, and nephelinite) have been studied using the fiber elongation method. Measurements were conducted in a stress range of 10–400 MPa and a strain rate range of 10−6 to 10−3 s−1. The stress‐strain rate relationships for all the melts exhibit Newtonian behavior at low strain rates, but non‐Newtonian (nonlinear stress‐strain rate) behavior at higher strain rates, with strain rate increasing faster than the applied stress. The decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt. The decrease in viscosity observed at the high strain rates of the present study ranges from 0.25 to 2.54 log10 Pa s. The shear relaxation times τ of these melts have been estimated from the low strain rate, Newtonian, shear viscosity, using the Maxwell relationship τ = ηs/G∞. Non‐Newtonian shear viscosity is observed at strain rates (
) equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time. Brittle failure of the fibers occurs 2 log10 units of time above the relaxation time. This study illustrates that the occurrence of non‐Newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate. High‐silica rhyolite melts involved in ash flow eruptions are expected to undergo a non‐Newtonian phase of deformation immediately prior to brittle failure.
The first direct measurements of upper oceanic crustal compressional wave attenuation
Authors: Jacobson R. S. , Lewis B. T. R.
Published: February 2008

The first direct measurement of compressional wave attenuation of the uppermost 650 m of oceanic crust was performed using data recorded by seafloor hydrophones and large (56–116 kg), deep, explosive sources. The site was 13 km east of the southernmost Juan de Fuca Ridge on crust 0.4 m.y. old Spectral ratios were performed between bottom refracting waves and direct water waves, adjusted for spreading losses and transmission coefficient losses. Several tests of the data were performed, demonstrating that attenuation is linearly related to frequency between 15 and 140 Hz, but frequency‐independent components of attenuation are also evident. Values of compressional wave Q cluster between 20 and 50 and do not show any systematic variation with depth over 650 m. The attenuation results also indicate the presence of heterogeneities within the crust, as the solutions for each receiver's data set are significantly different. No evidence for azimuthal variations of attenuation are supported by the data, although the data do not optimally sample a wide variation of azimuths. Our attenuation values are judged to be normal to higher than expected for the whole oceanic crust, based upon comparisons to results from synthetic seismogram modeling by others and by modeling signal to noise ratios of typical seismic refraction profiles. The results are consistent with recent laboratory measurements at ultrasonic frequencies for dry and saturated basalts at seafloor pressures and temperatures.
Compressional wave attenuation in oceanic basalts
Authors: Wepfer William W. , Christensen Nikolas I.
Published: February 2008

To understand better the seismic attenuation in the upper volcanic regions of the oceanic crust, compressional wave attenuations of oceanic basalts have been measured as a function of confining pressure using an ultrasonic pulse‐echo spectral ratio technique capable of measuring attenuations to pressures of 500 MPa. Seven basalts, five from Deep Sea Drilling Project cores and two from dredge samples, have wide ranges of densities, porosities, and alterations, making possible an analysis of the parameters influencing basalt attenuation. Attenuation increases with the volume of secondary minerals present and with increasing porosity. Thus vesicularity and compositional changes associated with basalt alteration will produce variations in attenuation. With the application of hydrostatic pressure, cracks close, thereby reducing attenuations. This pressure dependence should be manifested in oceanic layer 2 by decreasing attenuation with depth. An inverse relationship between velocity and attenuation is observed at high hydrostatic pressures. Water saturation increases attenuation at pressures below 200 MPa and enhances the sensitivity of attenuation to pressure, thus making the state of saturation important in the 40 to 100 MPa range generally found in layer 2. These results provide a framework for interpreting marine attenuation data.
The paleomagnetism of red beds and basalts of the Hettangian Extrusive Zone, Newark Basin, New Jersey
Authors: Witte William K. , Kent Dennis V.
Published: February 2008

A high unblocking temperature component of magnetization residing in hematite was isolated in 11 outcrop sites and 4 boreholes from Hettangian sedimentary rocks interbedded with the igneous extrusives of the Newark basin. This normal polarity characteristic magnetization has a mean direction (declination = 6.3°, inclination = 12.9° after tilt correction) which is significantly shallower (by about 15°) than reported magnetization directions from the coeval basalts. Based on our demagnetization analysis of representative basalt samples, we attribute the difference between the sedimentary and basaltic directions to contamination of some of the basalt magnetizations by a steeper overprint. An intermediate unblocking temperature magnetization recovered from the extrusive zone sedimentary rocks, consistent with evidence elsewhere in the Newark basin for a pervasive hydrothermal and remagnetization event at about 175 Ma, is a likely cause of the basalt contamination. The characteristic magnetization from the Hettangian sedimentary rocks yields a paleopole (55.5°N, 94.6°E, dp = 5.2°, dm = 5.4°) which lies on the westbound, precusp portion of current paleomagnetic Euler pole apparent polar wander (APW) paths, in contrast with the Newark (and Hartford) basin basalt reference pole positions (such as the “Newark trend” N1 pole) which lie on the eastbound, postcusp portion of the APW path. A precusp pole position is more compatible with the relative age of the Hettangian extrusive zone pole with respect to the Sinemurian/Pliensbachian poles which define the cusp but implies large changes in the rate of APW in the Late Triassic and Early Jurassic.
Origin of microgranular enclaves in granitoids: Equivocal Sr‐Nd Evidence From Hercynian Rocks in the Massif Central (France)
Authors: Pin C. , Binon M. , Belin J. M. , Barbarin B. , Clemens J. D.
Published: February 2008

We examined the Rb‐Sr and Sm‐Nd systematics of three different, high‐level to subvolcanic granitoid plutons and their microgranular enclaves. The host granitoids are adamellites and grano‐diorites, while the enclaves are mostly microtonalites with igneous textures. The enclaves are rounded in shape and vary from <10 cm to >1 m across. Contacts with the enclosing granitoids are sharp and, mostly, nonlobate. The microtonalitic enclaves are similar in mineralogy to their hosts. Initial 87Sr/86Sr are all ≥0.7070 and initial εNd is −3.0 to −6.6 for both the enclaves and their hosts. This precludes a major input of an unmodified, mantle‐derived component, at the time of magma genesis. Nd isotopic contrasts between enclaves and hosts are 1 ε‐Nd‐unit at most. These data indicate broadly cogenetic origins for enclave‐host systems, though some enclaves have slightly less “crustal” signatures than the enclosing granitoids. There are several possible interpretations. (1) There may have been nearly complete isotopic homogenization between enclaves and host magmas derived from different starting parents. (2) The enclaves and granitoids could all have purely crustal origins. Enclaves might represent magmas derived from slightly less evolved protoliths or parental magma, or they could be concentrations of early formed crystals, with interstitial melt. (3) Basaltic magma, from the depleted mantle, could have thoroughly mixed with a large volume of crustal melt, in the lower crust. (4) There could have been a significant input of melt derived from an enriched mantle source. Several of these mechanisms may have played roles in the genesis of the granitoids and their enclaves. Nd isotopes do not always allow unequivocal recognition of mantle components in granitoid magmatism. We argue that enclaves of this type are probably not products of major mixing processes relevant to granitoid protoliths and large‐scale crustal evolution. They commonly result from second‐order mingling of broadly cogenetic magma fractions that may have a variety of different origins.
Crystallization and hybridism in microgranitoid enclave magmas: Microstructural evidence
Authors: Vernon R. H.
Published: February 2008

Microstructural evidence indicates that many microgranitoid enclaves (“mafic inclusions”) in granitoids crystallize from globules of magma that have undergone previous magma mixing (hybridism). The evidence includes (1) rounded xenocrysts (“ocelli”) of quartz rimmed with fine‐grained aggregates of early formed minerals, especially mafic minerals; (2) K‐feldspar megacrysts, commonly rimmed with plagioclase (rapakivi structure); and (3) corroded cores, dendritic overgrowths, and calcic “spikes” in plagioclase. Initial quenching of a magma globule in felsic magma involves rapid temperature equilibration, which causes a high rate of nucleation of minerals precipitating from the enclave magma at the time, followed by a period of slow cooling, during which the nucleation and growth rates of later‐crystallizing minerals are similar to the much slower rates prevailing in the granitic melt. This may result in markedly poikilitic microstructures, involving inclusion of the earlier‐formed, finer‐grained minerals in quartz and/or K‐feldspar, which are common in microgranitoid enclaves.
Groundwater dilution and residence times, and constraints on chloride source, in the Mokai Geothermal System, New Zealand, from chemical, stable isotope, tritium, and 36Cl data
Authors: Hedenquist Jeffrey W. , Goff Fraser , Phillips Fred M. , Elmore David , Stewart Michael K.
Published: February 2008

The Mokai geothermal system, New Zealand, is located within a volcanic collapse structure comprised of flat‐lying silicic rocks. The deep chloride fluid rises in the south, inside the caldera margin; near the surface the fluid flows laterally more than 12 km to the north, because of the hydraulic gradient caused by 300 m of relief. The maximum measured temperature of the production wells is near 323°C at 2500 m depth, whereas temperatures in two wells on the northern margin of the upflow are <200°C, with thermal inversions at 350 and 1000 m indicating lateral flow. These two marginal wells have about 50% of the chloride of the deep fluid, indicating dilution by cold ground water. Chloride‐bearing springs discharge 6 km north of the zone of upflow and are even further diluted by cold groundwater than the wells on the margin of the system. The tritium data in this hydrologically rather simple system indicate that there is a mean residence time of greater than 100 years for the groundwater diluent; also, older water is mixed with <25% of young (thermonuclear, post‐1955) water before the mixture dilutes outflowing chloride fluid from the deep system. The deep chloride fluid has a 36Cl/Cl ratio of 2.6×10−15 and a 36Cl concentration of 106 atoms per 10−6 kg. This fluid has a lower ratio (by a factor of 90) and a higher absolute concentration (by a factor of 7) than present‐day stream water. However, the marginal wells have been diluted by groundwater with at least 500 36Cl atoms per 10−6 kg, 30 times richer in 36Cl than present‐day stream waters. This spike is most likely due to circulation of precipitation formed during thermonuclear testing to depths of 1000 m (though the component with the 36Cl spike only makes a moderate, <25%, contribution to the groundwater diluent; most of the diluent is much older, pre‐1955). The amount of 36Cl in the deep fluid at Mokai is 5–10 times less than that predicted from a calculation of the neutron fluxes in the silicic reservoir and underlying greywacke basement rocks. This indicates that the chloride was not derived from host rock leaching in the upper 3 km; rather, the fluids have interacted with more primitive rocks below the drilled depths of the Taupo Volcanic Zone, with the chloride possibly derived from a magmatic brine.
Quaternary extrusion rates of the Cascade Range, northwestern United States and southern British Columbia
Authors: Sherrod David R. , Smith James G.
Published: February 2008

Quaternary (2–0 Ma) extrusion rates change significantly along the Cascade Range volcanic arc. The extrusion rate north of Mount Rainier is about 0.21 km3km−1m.y.−1; the rate in southern Washington and northern Oregon south to Mount Hood is about 1.6 km3km−1m.y.−1; in central Oregon the rate is 3–6 km3km−1m.y.−1; and in northern California, the rate is 3.2 km3km−1m.y.−1. Eruption style also changes along the arc but at latitudes different from rate changes. At the ends of the arc, volcanism is focused at isolated intermediate to silicic composite volcanoes. The composite volcanoes represent ∼30% of the total volume of the arc. Mafic volcanic fields partly ring some composite volcanoes, especially in the south. In contrast, volcanism is diffused in the middle of the arc, where numerous overlapping mafic shields and a few composite volcanoes have built a broad ridge. Contrasting eruption style may signify diffuse versus focused heat sources or may reflect changes in permeability to ascending magma along the arc.
Stratigraphic, lithologic, and major element geochemical constraints on magmatic evolution at Lassen Volcanic Center, California
Authors: Clynne Michael A.
Published: February 2008

The Lassen volcanic center is the most recent of several long‐lived volcanic centers in the southernmost Cascade Range. These centers have erupted products ranging from basaltic andesite to rhyolite and are superimposed on a background of regional basaltic to andesitic volcanism. The evolution of the Lassen volcanic center is described in three stages. Stages I and II comprise the Brokeoff volcano, and 80 km3 andesitic stratocone, active from 600 to 400 ka. Brokeoff volcano is compositionally equivalent to the regional basaltic andesite to andesite volcanism in the Lassen region and is the result of structurally controlled focusing of the diffuse regional mafic magmatism. Stage III comprises a silicic dome field and adjacent area of hybrid andesites and has a total volume of about 100 km3. Volcanism during stage III was episodic and is subdivided into four sequences of lithologically and temporally distinct lavas. Stage III began at 400 ka with a rhyolitic, caldera‐forming pyroclastic eruption and chemically related lavas. Additional sequences of dacite erupted between 250–200 ka and 100–0 ka. Hybrid andesites erupted adjacent to the silicic dome field between 300 and 0 ka. Porphyritic andesite and dacite with high Al2O3, low TiO2, medium K2O and FeO/MgO ratios of 1.5–2.0 are the most abundant rock types in the Lassen volcanic center. However, the single most voluminous unit is sparsely phyric rhyolite pumice. In general, the lavas of Lassen volcanic center form a single coherent trend on major element variation diagrams and in pseudo‐quaternary phase space, consistant with an origin either by fractional crystallization or magma mixing. In detail, however, the lack of systematic temporal change in silica and subtly crossing trends indicate a complex origin. A variety of statistically successful fractional crystallization models can be constructed that derive Brokeoff andesites from regional magmas. An important conclusion of the modeling is that if fractional crystallization is the process responsible for generation of Brokeoff andesite, then the parent magma must be low to medium K in geochemical affinity in order to explain the variation in K2O. However, although major element variation can be modeled by fractional crystallization, petrographic and stratigraphic evidence indicates that magma mixing is an important but subtle process in Brokeoff lavas and suggests that lavas evolved in small independent batches. Lavas erupted during stage III, while predominantly silicic, range from 53 to 75% SiO2. Disequilibrium mineral assemblages in the stage III lavas indicate that they are not directly derived from Brokeoff andesite by fractional crystallization. Mixing of silicic magma with regional mafic magma and disaggregation of andesitic quenched magmatic inclusions play dominant roles in the compositional diversity of stage III lavas.
An analysis of the electrode effect in the limit of strong turbulent mixing
Authors: Willett J. C.
Published: February 2008

A simplified version of the turbulent electrode effect is developed for use as a charge source in a convection current model. Asymptotic analysis is applied to the conservation equations for small ions to show that the conductivity profile in the electrode layer becomes independent of the conduction process in the limit of strong turbulent mixing. In this limit, which is found to be applicable to the atmospheric surface layer under many circumstances, the electrode effect can be described simply by a charge conservation equation with an externally determined mean conductivity profile decreasing to zero at the surface.
Radon 222 measurements during marine fog events off Nova Scotia
Authors: Larson R. E.
Published: February 2008

Radon (222Rn) measurements provided a simple, reliable, in situ indicator of the continental component of the resident air masses during marine fog events off Nova Scotia in August 1975. Frequent and substantial changes in the continental component of the air, including an increase from 5 pCi m−3 (mostly maritime air) to 80 pCi m−3 (continental air) within 6 hours on August 5 at a location 100 km off shore illustrate the variations in the nature of over‐ocean air and the importance of continuous sampling of the surroundings during shipboard atmospheric experiments. The recent 1‐ or 2‐day histories of the air for several fog events were investigated by relating the air mass movement as suggested by weather maps to the measured radon concentration. The reliability of determination of atmospheric radon concentrations by measurement of the beta radioactivity of daughter products collected on filter paper was verified as being applicable in fog.
The effect of internal waves on surface wind waves 1. Experimental measurements
Authors: Hughes B. A. , Grant H. L.
Published: February 2008

Internal waves were generated by a ship using the ‘dead water’ effect in areas where the water contains a strong near‐surface density gradient. The effects of these internal waves on wind waves were examined. The principal measurements were slope statistics of the wind waves and horizontal currents in the internal waves. The effects on the wind waves were always observable from an aircraft; however, in measurements made only along the ship's track the effects of the internal waves were not always readily distinguishable from other factors that influence the wind wave field. By using statistical techniques, relationships have been established between the wind waves, the internal waves, and the wind velocity. The principal finding is that the wind wave field is relatively more sensitive to internal wave currents at low wind speeds than at high wind speeds. Numerical values are given.
Light scattering from particles of different shapes
Authors: Gibbs Ronald J.
Published: February 2008

The light scattering intensities of white light, at various scattering angles, from glass spheres, crushed quartz, and mica flakes in suspension in water were measured. The light scattering distributions for the various shapes in the angular region from 0° to 90° were similar; however, in the region from 90° to 140° the distributions were quite different. The spheres produced the least scattering of light, and the plate‐shaped particles produced the greatest scattering of light in this region. A technique is proposed for the determination of shape utilizing light scattering by use of the ratio of light scattered at 90° to that scattered at 140°.
Effects of nonmethane hydrocarbons in the atmosphere
Authors: Chameides W. L. , Cicerone R. J.
Published: February 2008

The photochemistry of several unreactive and moderately reactive nonmethane hydrocarbons (NMHC) in the background troposphere and stratosphere was investigated. A one‐dimensional steady state model was employed to determine the vertical distributions of C2H6, C2H2, C3H8, C4H10, and C5H12. The impact of these species upon the tropospheric and stratospheric odd‐hydrogen, odd‐oxygen, chlorine, and carbon systems was studied for various possible free radical profiles and eddy diffusion coefficients. Our results indicate that NMHC probably have only a small effect upon the background atmospheric photochemistry, although they might constitute a nonnegligible source of atmospheric CO. Also, Cl atoms, in predicted present‐day concentrations, comprise the major sink for stratospheric NMHC. Finally, if the chlorovinyl molecule (CHCl = CH) were stable in the lower stratosphere, it would then be conceivable that C2H2 could be partially effective as a chain terminator to impede catalytic removal of stratospheric O3 by Cl and ClO.
The effect of increased carbon dioxide concentrations on stratospheric ozone
Authors: Boughner R. E.
Published: February 2008

During the past several years, much attention has been focused on the destruction of ozone by anthropogenic pollutants such as the nitrogen oxides and chlorofluoromethane. Little or no attention has been given to the influence on ozone of an increased carbon dioxide concentration for which a measurable growth has been observed. Increased carbon dioxide can indirectly affect ozone by perturbing atmospheric temperatures, which will alter ozone production, whose rate displays a fairly strong temperature dependence. This paper presents one‐dimensional model results for the steady state ozone behavior when the CO2 concentration is twice its ambient level which account for coupling between chemistry and temperature. When the CO2 level doubled, the total ozone burden increased in relation to the ambient burden by 1.2–2.5%, depending on the vertical diffusion coefficient used. Above 30 km, ozone concentrations were larger than the ambient values, a maximum increase of 16% being reached at 43 km. In this region the relative variations were insensitive to the choice of diffusion coefficient. Below 30 km, ozone concentrations were smaller than the unperturbed values and were sensitive to the vertical diffusion profile in this region (10–30 km). Ozone decreases in the lower stratosphere because of a reduction in ozone‐producing solar radiation, which results in smaller downward ozone fluxes from the region at 25–30 km relative to the flux values for the ambient atmosphere. These offsetting changes occurring in the upper and lower stratosphere act to minimize the variation in total ozone.
Edge waves and surf beat
Authors: Bowen Anthony J. , Guza Robert T.
Published: February 2008

Surf beat, wave motion at relatively low frequency (periods of 30–200 s), is often observed on beaches. However, even with modern instrumentation it is difficult to describe the spatial variation of the low‐frequency motion; consequently, the relative importance of a number of suggestions which, at least in theory, provide mechanisms for the generation of low‐frequency energy has never been established. Recent observations (e.g., Huntley, 1976) have reinforced the idea that edge waves, the free wave modes trapped at the shoreline, are a major component of low‐frequency energy. One of the most interesting explanations of surf beat suggests that the beating between particular pairs of incoming waves leads to resonant growth of edge wave modes, which may then dominate the low‐frequency spectrum (Gallagher, 1971). Empirical evidence is essential, as any theoretical development breaks down when the incoming waves break, a fundamental problem with Gallagher's (1971) model. To investigate the importance of this resonant interaction, the general interaction conditions were therefore used to design laboratory experiments in which both resonant and nonresonant conditions were expected. The experimental results show that the response at the beat frequency is stronger when the resonance conditions for edge wave growth are satisfied and that the response is in the form of the theoretically predicted edge wave mode, even when the incident waves are breaking. These results strongly suggest that surf beat is predominantly an edge wave phenomenon.
Uncertainty propagation in a stratospheric model 1. Development of a concise stratospheric model
Authors: Rundel R. D. , Butler D. M. , Stolarski R. S.
Published: February 2008

Analysis of uncertainties in stratospheric perturbations requires the development of a model which uses a minimum of computer time yet is complete enough to represent reasonably the results of more complex models. Such a concise model has been developed and is described in this paper. The model is a steady state model using iteration to achieve coupling between interacting species. The species odd oxygen, H2O, H2, CO, N2O, odd nitrogen, CH4, CH3Cl, CCl4, CF2Cl2, CFCl3, and odd chlorine are determined from diffusion equations with appropriate sources and sinks. The HOx species, methane oxidation products, and relative species abundances within the odd oxygen, odd nitrogen, and odd chlorine families are solved in photochemical equilibrium, and N2, O2, CO2, and temperature have been assigned fixed profiles. Diurnal effects such as those due to chlorine nitrate formation are accounted for by analytic approximations. The model has been used to evaluate steady state perturbations due to injections of chlorine and NOx. The results are similar to those obtained by other models.
Effect of multiple scattering on species concentrations and model sensitivity
Authors: Luther Frederick M. , Wuebbles Donald J. , Duewer William H. , Chang Julius S.
Published: February 2008

Multiple scattering significantly affects atmospheric photodissociation rates, particularly at wavelengths greater than 290 nm. Incorporating the effects of multiple scattering into a one‐dimensional transport‐kinetics model led to dramatic changes in the concentration profiles of many trace species; thus it is important to include multiple scattering when model results are compared with observational data. The effect on model sensitivity of including multiple scattering ranged from no change in ΔO3 compared with the pure absorption calculation to a significant increase in ΔO3 depending upon the model chemistry. The change in ΔO3 due to multiple scattering was small in comparison with the change in ΔO3 resulting from varying chemical reaction rates. Variations in surface albedo had little effect on model sensitivity.
Effects of a coastal front on the distribution of chlorophyll in Lake Tahoe, California‐Nevada
Authors: Leigh‐Abbott Mark R. , Coil John A. , Powell Thomas M. , Richerson Peter J.
Published: February 2008

The existence of a distinct coastal zone is confirmed in Lake Tahoe (California‐Nevada) by horizontal transects measuring chlorophyll and temperature simultaneously. Creation of the coastal region is influenced by bottom topography, the nature of the surface wind stress, and the difference between physical processes occurring within a Rossby radius of deformation and those occurring in midlake. Chlorophyll records from horizontal transects were decomposed by spectral analysis, and the normalized spectra from nearshore and midlake were compared. The two regions were found to differ at large scales, primarily because of differences in nutrient import, and at intermediate scales because of differences in the mixing regime. The coastal zone was observed to erode with increasing winds and weakening stratification, leading to little significant difference between nearshore and midlake chlorophyll patterns.