bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxygen diffusion in San Carlos olivine
Authors: Gérard O. , Jaoul O.
Published: February 2008

The self‐diffusion of oxygen in San Carlos olivine single crystals has been measured along the [001] direction. The diffusion experiments were performed using a gas mixture Ar + H2/Ar + H218O flowing around the olivine specimens. This gas mixture acts as the isotopic reservoir and is also used to establish and adjust the desired oxygen fugacity, allowing us to determine the oxygen diffusion coefficient D as a function of pO2. Thirty runs have been performed at 1 atm total pressure within the (T,pO2) olivine stability field, T ranging from 1363 to 1773 K and pO2 from 7×10−7 to 5×10−2 Pa. The annealing duration t was chosen such that the characteristic isotope penetration depth
ranges between 600 and 11,000Å. The diffusion profiles of 18O were determined using the 18O(p, α)15N nuclear microanalysis technique. The diffusion coefficient can be written D* = D0* exp(−E/RT)(pO2/p0)m, where E is the activation energy, R is the gas constant, T is the temperature in K, pO2 is the oxygen partial pressure, p0 is the room pressure, m is a numerical coefficient, and D0* is the preexponential term. A data inversion method has been used to determine the three parameters and yields log(D0*) = −5.2(±3.6) with D0* in m2/s, E = 318(±17) kJ/mol, m = 0.34(±0.02); D* = 6.7 × 10−6exp(−38,000/T)(pO2/p0)0.34 m2/s; the number in parentheses is the standard deviation. D* is related to the self‐diffusion coefficient D by the relation D* = ƒD, with ƒ close to 1.0. An attempt to show pipe diffusion in predeformed samples, with a dislocation density of about 2×1011 m−2, along dislocation cores perpendicular to the specimen surface, showed no contribution to the main bulk diffusion. We conclude that in olivine, Fo≃90, within the temperature range investigated, DFe, DMg ≫ D0x ≫ DSi as has already been demonstrated in FO100. E is of the same order of magnitude as the Si ‐ O bond energy and as E in Fo100. We infer that the oxygen diffusion occurs via an interstitial mode and that these defects carry one negative charge. This interstitial mechanism is rather surprising in view of the compact oxygen sublattice of olivine. However, interstitial transport of oxygen has already been identified in other oxygen compact compounds.
Constraints on the abundance of eclogite in the upper mantle
Authors: Schulze Daniel J.
Published: February 2008

Examination of heavy mineral concentrates from several well‐known eclogite‐dominated kimberlites (Roberts Victor, Bobbejaan, Zagadochnaya) has revealed that the upper mantle sampled by these kimberlites was dominantly peridotite, with only a small amount of eclogite (3–15% by volume). In the case of Group II (micaceous) kimberlites in southern Africa (e.g., Roberts Victor), it is proposed that garnet peridotites were metasomatized (phlogopitized) within the upper mantle prior to xenolith incorporation. This caused structural weakening, with the result that most of the garnet peridotites disaggregated during kimberlite ascent and eruption, leaving an eclogite‐dominated xenolith suite. The amount of eclogite in the upper 200 km of the subcontinental upper mantle is concluded to be perhaps <1% by volume overall. Support for models proposing a significant amount of eclogite at deeper levels or beneath ocean basins must be sought from evidence other than mantle‐derived xenoliths.
Structural analysis of the Southern Peninsular, Southern Wrangellia, and Northern Chugach Terranes Along the Trans‐Alaska Crustal Transect, Northern Chugach Mountains, Alaska
Authors: Nokleberg Warren J. , Plafker George , Lull John S. , Wallace Wesley K. , Winkler Gary R.
Published: February 2008

Structural and tectonic analysis of the southern Peninsular, southern Wrangellia, and northern Chugach terranes, along the Trans‐Alaska Crustal Transect in the northern Chugach Mountains documents a long succession of Early Jurassic through Cenozoic deformational events. The deformational events are generally characterized by distinctive structural fabrics and metamorphisms. Most of the events are interpreted to be related to subduction‐related accretion or terrane accretion along the Border Ranges fault system (BRFS) and companion faults, and the Contact fault system (CFS). Each period of subduction‐related accretion consisted of underplating of the outboard unit beneath the adjacent inboard unit. The fabric associated with each subduction‐related accretion consisted of folding, intense shearing, and local rolling of planar structures. Age and structural relationships suggest migration of the zone of subduction‐related accretion from the BRFS to the north, through each accreting unit, to younger bounding thrust faults to the south. Other older and younger deformational events are also recognized and are interpreted to have formed before and after, respectively, accretions along the BRFS and CFS. The main younger deformational events are (1) early Tertiary north verging folding of portions of the northern Chugach and southern Wrangellia terranes and (2) broad folding and rotation of major and minor structures related to subduction‐related accretion or terrane accretion during early and middle Tertiary oroclinal bending of Alaska.
Hybridization processes in leucite tephrites from Vulsini, Italy, and the evolution of the Italian potassic suite
Authors: Varekamp Johan C. , Kalamarides Ruth I.
Published: February 2008

A stratigraphic sequence of scoria samples from a cinder cone in Vulsini, Italy, was studied for bulk rock and mineral chemistry and oxygen, strontium, and neodymium isotope chemistry. Hybridization processes between a relatively unevolved leucititic magma and an evolved trachytic magma of the low‐K series are indicated by the mineral paragenesis, bulk rock trace element signature, mineral chemistry, oxygen isotope ratios of the crystals, and composition of glass inclusions. The assimilation of pelitic material is suggested by the presence of cordierite‐bearing glass blobs. Mixing calculations demand at least four end‐members to explain the spectrum of compositions. Our detailed data show a variation in oxygen isotope composition comparable to the whole range of values obtained from Vulsini rocks. We propose a model of magmatic evolution in a chemically and thermally open system, in which evolved trachytic magmas have assimilated substantial amounts of crustal material with a heat supply from an underlying, primitive leucititic magma. Mixing of these strongly contaminated magmas with the underlying slightly evolved leucititic magmas led to the leucite tephrites. Limestone assimilation and absorption of high δ18O geothermal waters might explain the poor correlation between stable isotope systematics and bulk rock chemistry. The δ18O value of an unevolved leucititic magma represented in the scoria samples is slightly above the common range of mantle‐derived magmas.
An experimental study of hydroxyl in quartz using infrared spectroscopy and ion microprobe techniques
Authors: Rovetta M. R. , Blacic J. D. , Hervig R. L. , Holloway J. R.
Published: February 2008

We have measured the concentrations of hydroxyl, deuterium, Al, Fe, Li, Na, K, and Rb in a natural quartz crystal before and after hydrothermal treatment at 1.5 GPa and 800°–1050°C. We employed microbeam infrared spectroscopy and ion probe techniques to avoid impurities trapped in healed cracks and fluid inclusions that might bias a normal bulk analysis. The ƒH2 of our experiments were buffered to the hematite‐magnetite‐(OH) fluid, nickel‐nickel oxide‐(OH) fluid, or iron‐wustite‐(OH) fluid phase assemblages. After hydrothermal treatment, the samples contained local concentrations of hydrogen or deuterium of several hundred atoms/106 Si (the starting crystal contained 45 H/106 Si). We did several experiments with Al2O3 or RbCl added to the sample charge and found local Al enrichment where the deuterium concentration was high but no Rb enrichment. Finally, we measured trace elements and hydroxyl in a quartz sample after plastic deformation in a talc furnace assembly; in regions of the sample containing basal and prismatic deformation lamellae (but no visible healed microcracks at 400× optical magnification) hydroxyl had increased to ∼200 OH/106 Si with no increase in Al or Fe. Samples enriched in hydroxyl but not Al (including the plastically strained sample) gave infrared spectra resembling natural amethyst crystals. We observed that the sharp pleochroic peaks near ∼3400 cm−1 and present in the starting crystal, were very intense only in samples showing Al enrichment, whereas the intensity of the sharp pleochroic peaks near 3600 cm−1 and broad isotropic absorption were independent of Al. Our analyses indicate that more hydrogen was introduced into the treated samples than Al or Fe. Because one proton or alkali cation is needed to screen each Al or Fe atom substituted into a Si lattice site, we conclude that the hydrothermal treatment had produced new hydroxyl defects in the quartz that did not contain Al or Fe. Although the speciation of this excess hydroxyl is unknown, it is present in all varieties of quartz that show hydrolytic weakening: synthetic quartz, amethyst, hydrothermally treated natural quartz crystals, and natural quartz deformed in talc assemblies. In the absence of microcracking or solution‐precipitation mechanisms that may mechanically trap OH or H2O molecules, we suggest that hydrogen diffusion, and reaction with lattice oxygen, may introduce hydroxyl defects into quartz and contribute to hydrolytic weakening.
Mapping high Pn velocity beneath the Colorado Plateau constrains uplift models
Authors: Beghoul Noureddine , Barazangi Muawia
Published: February 2008

The massive International Seismological Centre data set of the past 20 years and the two‐station method are used to determine Pn velocities in the mantle lid beneath the Colorado plateau. In this method the event is located at distances where Pn is the first arrival (2°–16°) and the path is in or very near the azimuth of the two‐station pair and crosses the plateau. This technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source, since only the difference in travel times at the two stations is used. However, this technique has a few underlying assumptions and possible sources of errors (such as the quality of the Pn data base and station delays caused by varying crustal structure) that require an extremely careful application of the method. A detailed study of the source of errors and a methodology of selection of the data are presented. Application of this method to the Colorado plateau using all possible two‐station pairs from 53 stations located within or along the margin of the plateau yields an average high Pn velocity of 8.12±0.09 km/s. This value is considerably larger than the average value of 7.83 km/s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8.1 km/s for the relatively stable midcontinent region. Our new Pn velocity for the Colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average Pn velocities beneath the Colorado plateau and the Basin and Range Province while their tectonic and magmatic Cenozoic history is dramatically different. Previous models for the structure and evolution of the plateau have used the low Pn velocity as an important constraint on density and thermal state of the lithosphere. Hence such models should be reexamined on the basis of this new uppermost mantle Pn velocity determination. There are two main models that have been proposed to explain the 2‐km uplift of the Colorado plateau. One is based on a combination of thermal thinning of the lithosphere and crustal thickening, and the other involves a combination of the delamination of the subducted, subhorizontal Farallon oceanic plate from the overriding North American plate and crustal thickening. We show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the Cenozoic evolution of western North America.
Appalachian Stress Study: 1. A detailed description of in situ stress variations in Devonian shales of the Appalachian Plateau
Authors: Evans Keith F. , Engelder Terry , Plumb Richard A.
Published: February 2008

We describe an experiment to measure variations in the state of stress within a horizontally bedded Devonian shale/sandstone/limestone sequence in western New York. A total of 75 stress measurements were made in three wells a kilometer or so apart using a wireline‐supported hydraulic‐fracturing system. The stress profiles indicate that a major drop in horizontal stress level occurs in the generally massive shales. This drop occurs principally across the lowermost member of a group of sand beds and corresponds to an offset in Sh and SH of 3.5 and 9 MPa, respectively. Above the sands, “thrust” regime conditions prevail, although the amount by which Sh exceeds Sv is undetermined since instantaneous shut‐in pressures (ISIPs) were clipped at the level of Sv due to fracture rotation. Below the sands, the regime is strike slip with both horizontal stresses showing lateral uniformity despite substantial variations in topography. The magnitude of Sh in the sand beds themselves and a lower limestone remains at least as great as Sv despite the decline in shale stress. Hence stress contrasts between these beds and neighboring shales become pronounced with depth. The contrast in Sh and SH between the lowermost sand and the immediately underlying shale is at least 6 and 14.5 MPa, respectively. SH levels in the lower strike‐slip regime are about 1.75 times greater than Sh and are less than the value required to initiate slippage on favourably oriented frictional interfaces. For the upper thrust regime and the sand and limestone beds, however, the inferred lower bound on SH is close to the slippage threshold for a Coulomb friction coefficient of 0.6. The orientation of SH is ENE with a standard deviation of 20°. Fracture traces were usually splayed, occasionally spanning 30° of well bore. No systematic correlation between mean orientation and lithology is evident. Significantly different orientations were obtained for adjacent tests in which almost identical ISlPs were observed, suggesting that the fractures quickly reorient themselves to propagate normal to the least principal stress direction. Similarly, vertical traces were observed in those tests where ISIPs apparently reflect Sv, suggesting that rotation to horizontal was rapid.
Comparison of seismic waveform inversion results for the rupture history of a finite fault: Application to the 1986 North Palm Springs, California, earthquake
Authors: Hartzell Stephen
Published: February 2008

The July 8, 1986, North Palm Springs earthquake is used as a basis for comparison of several different approaches to the solution for the rupture history of a finite fault. The inversion of different waveform data is considered; both teleseismic P waveforms and local strong ground motion records. Linear parametrizations for slip amplitude are compared with nonlinear parametrizations for both slip amplitude and rupture time. Inversions using both synthetic and empirical Green's functions are considered. In general, accurate Green's functions are more readily calculable for the teleseismic problem where simple ray theory and flat‐layered velocity structures are usually sufficient. However, uncertainties in the variation in t* with frequency most limit the resolution of teleseismic inversions. A set of empirical Green's functions that are well recorded at teleseismic distances could avoid the uncertainties in attenuation. In the inversion of strong motion data, the accurate calculation of propagation path effects other than attenuation effects is the limiting factor in the resolution of source parameters. The assumption of a laterally homogeneous velocity structure is usually not a good one, and the use of empirical Green's functions is desirable. Considering the parametrization of the problem, any degree of fault rupture complexity can be described in terms of a linear parametrization for slip amplitudes. However, a nonlinear parametrization for rupture times and slip amplitudes can have a distinct advantage over a simple linear one by limiting the number of unknown parameters. Regardless of the choice of data or the type of parametrization, the model or solution will be affected by the choice of minimization norm and the type of stabilization used.
A dynamic model for fluid flow in open borehole fractures
Authors: Tang X. M. , Cheng C. H.
Published: February 2008

The attenuation of Stoneley waves by an open borehole fracture is closely related to fluid flow in the fracture. We consider the dynamic response of a viscous fluid in a borehole fracture to the oscillating pressure excitation of Stoneley waves at the fracture opening. A dynamic model has been developed to describe the fluid motion in the fracture. This model relates both viscous shear effects at the fracture surface and the acoustic wave propagation in the fracture fluid. The dynamic conductivity is derived to characterize the fluid conduction through the fracture. This model is applied to study Stoneley wave attenuation across a horizontal and along a vertical borehole fracture. In the horizontal fracture case, the theory is found to agree with laboratory fracture modeling data after minimizing scattering effects due to the fracture. In the vertical fracture case, where the scattering effects are minimal, the theory is in excellent agreement with the laboratory data.
Single‐crystal elastic constants for San Carlos peridot: An application of impulsive stimulated scattering
Authors: Brown J. M. , Slutsky L. J. , Nelson K. A. , Cheng L.‐T.
Published: February 2008

The utility of laser‐induced phonon spectroscopy in the determination of the elastic constants of small single crystals is demonstrated in the case of olivine of Fo89 composition. Experimental precision in the acoustic velocities is approximately 0.2%. The elastic constants derived in the current experiments have formal uncertainties of less than 1%. In analysis of all olivine data with compositions between Fo0 and Fo100, diagonal elastic constants are linear in composition to better than 1%, while off‐diagonal constants exhibit scatter of several percent. Preliminary data at high pressure indicate that this technique can be applied to studies of mineral elasticity under extreme conditions.
Absorption and modulus measurements in the seismic frequency and strain range on partially saturated sedimentary rocks
Authors: Paffenholz Josef , Burkhardt Hans
Published: February 2008

The absorptions 1/QE and 1/QS of partially water‐saturated sedimentary rocks were determined from phase differences between stress and strain for longitudinal deformation frequencies between 0.03 and 300 Hz and torsional frequencies between 0.03 and 100 Hz. Both longitudinal and shear strain amplitudes were of the order of 10−6. For water saturation between 0 and 50%, Young's moduli and shear moduli were shown to decrease with increasing saturation. In this saturation range both 1/QE and 1/QS increased, but no distinguishable absorption maxima were observed. For saturation percentages greater than 50%, the moduli of the samples appeared independent of the water content. 1/QE increased until full saturation and showed a strong frequency dependence. 1/QS is much less affected by increasing water saturation and has no absorption maxima in general. Since the absorption and the moduli reduction show different saturation dependencies, two different mechanisms for these effects are proposed. The addition of water changes the interaction force between the molecules on opposite walls of thin cracks. This provides a mechanism for the modulus reduction. Calculations based on the Biot‐Gardner theory (Gardner, 1962) indicate that part of the observed absorption may be caused by fluid flow due to the limited sample size as proposed by White (1986). Fluid‐supported thermorelaxation is proposed, as a possible intrinsic absorption mechanism.
Effects of lateral viscosity variations on long‐wavelength geoid anomalies and topography
Authors: Richards Mark A. , Hager Bradford H.
Published: February 2008

Models for the Earth's geoid and dynamically supported topography usually account only for radial (vertical) variations in mantle rheology and flow structure. However, expected lateral variations in viscosity, due to temperature‐ or stress‐dependent rheology, can degrade our ability to infer mantle structure from geoid models based on seismically determined density contrasts in the mantel. Theoretically, the main difficulty is that the harmonic components of the flow‐stress‐deformation field are no longer independent, and exact analytical methods cannot account for this “mode coupling.” For large thermal density contrasts of small lateral dimension (e.g., subducted slabs), it is difficult to form any general theory for these effects. However, we believe that the largest geoid anomalies which occur at the lowest harmonic degrees (l = 2,3) are due to a global pattern of relatively mild temperature variations. For convection with mainly broadscale thermal and rheological changes, some relatively straightforward ideas can be developed. Combining results from simple perturbation theory and numerical models for convective flow, we find the following: (1) If we are correct in inferring viscosity variations of less than one order of magnitude globally, the longest‐wavelength geoid anomalies (l = 2,3) are not seriously contaminated by lateral viscosity variations. (2) Contamination of the higher degrees (l≥4) is expected, and, in particular, self‐coupling between l = 2 heterogeneity and viscosity variations may be observable at degree 4; the most severe contamination occurs at the doubled harmonic (l = 4) of the dominant heterogeneity harmonic (l = 2). (3) Perturbation techniques are adequate to model global‐scale lateral viscosity variations of an order of magnitude or less. (4) The steady state surface deformation problem at the heart of our geoid models may be more robust with respect to broadscale lateral viscosity variations than are models for transient deformations (e.g., postglacial rebound). (5) For loading by broadscale density heterogeneity, the effects of stress‐dependent rheology are probably secondary to the effects of radial viscosity stratification.
Conductive structures and tectonics beneath the EMSLAB land array
Authors: Gough D. I. , McKirdy D. M. , Woods D. V. , Geiger H.
Published: February 2008

Magnetovariation fields were recorded by the land part of the EMSLAB array, August‐September 1985. Data from two complete days and two shorter sequences, chosen for moderate magnetic activity, are here used to map induced currents, which flow preferentially in the more conductive rocks, in Washington and Oregon and adjoining areas. Fourier transform anomaly maps, induction vectors, and hypothetical event anomaly maps are used to delineate conductive structures. These include the ocean and suboceanic asthenosphere and extended regions of attenuated vertical component caused by highly conductive layers under the Basin and Range Province and the Canadian Cordillera, both considered associated with partial melting and hydrothermal water. The Cascade Range of volcanoes has a north‐south strip of high conductivity some 100 km wide beneath it, which is named the Deep Cascades conductor. Between 46° and 47°N the Deep Cascades currents appear to flow up through a narrow upper crustal conductor, well known from prior work by others, into Puget Sound. The Deep Cascades conductor correlates with high heat flow and probably delineates the partial melting beneath the volcanic arc. At its southern end it appears to be continuous with the Basin and Range regional conductor. Northward it terminates between 47.5° and 48°N, south of Mounts Baker and Garibaldi. The Blue Mountains of NE Oregon and SE Washington coincide approximately with a kidney‐shaped block of resistive lithosphere, which lies between the Klamath‐Blue Mountain Lineament and the conductive mantle of the Basin and Range. The Columbia Embayment and plateau basalts occupy a triangular block of lithosphere more resistive than the Canadian Cordilleran Regional Conductor, which abuts against it, but less resistive than the Blue Mountains. Boundaries between these tectonic units are mapped from the associated magnetovariation anomalies, that is, in terms of electrical conductivity.
Fracturing process of granite inferred from measurements of spatial and temporal variations in velocity during triaxial deformations
Authors: Yukutake H.
Published: February 2008

Fracturing in fine‐grained homogeneous granite during triaxial deformation was investigated by measuring the spatial and temporal variations of elastic wave velocities. Velocities along 20 ray paths were measured with 12 piezoelectric transducers attached to a sample under confining pressures up to 400 MPa. Systematic velocity differences observed among the 20 paths apparently indicate the formation of a dilatant region near the surface of a sample. The damaged region formed near the surface due to asymmetric development of dilatancy seemed to play an important role in controlling ultimate failure. No evidence of the formation of a crack concentration zone adjacent to the ultimate fracture plane could be found before failure, insofar as could be estimated from the velocity variations of elastic waves propagating along its plane. However, a highly concentrated region of cracks in which the fracture nucleated was generated in the damaged region at stresses of more than 97–99% of the ultimate strength in all the specimens based on tomographic images computed using an algebraic reconstruction technique. The elastic waves were strongly affected by the damaged region even under the high confining pressures at which cracks are considered to close.
Heat flow in the Great Plains of the United States
Authors: Gosnold William D.
Published: February 2008

Anomalous high heat flow previously reported for the Great Plains is inconsistent with the tectonic setting and requires reexamination. Forty‐six new heat flow measurements, 12 revised heat flow values, and several hundred geothermal gradient measurements indicate extensive geothermal anomalies with heat flows ranging from 80 to 140 mW m−2 in the northern and central Great Plains. Heat flow in the Great Plains outside the geothermally anomalous regions ranges from 40 − 60 mW m−2. The heat flow anomalies result from the thermal effects of regional groundwater flow where it moves upward either within a dipping aquifer or by cross‐formational flow through fractures. The gravitational driving force for the groundwater flow derives from the eastward sloping surface of the Great Plains, and the locations of the geothermal amonalies are determined by the structures of the aquifers and the crystalline basement rocks. The most widespread and largest‐amplitude geothermal anomaly occurs in southern South Dakota and northern Nebraska. Another large anomaly occurs on the eastern flank of the Denver Basin, and small anomalies occur on structures such as the Billings and Nesson anticlines in the Williston Basin. Previous reports of high heat flow in the Great Plains generally are supported by the results of this study. However, the source of anomalous heat is shown to be nontectonic, and theoretical arguments for normal continental heat flow in the Great Plains are supported. Another difference from the results of previous heat flow studies is that the thermal conductivities of shales in the Mesozoic strata in the Great Plains are about 40% lower than the conductivities that commonly have been used for shales. This observation and recent studies which have suggested lower thermal conductivities for shales in the Great Plains are the reasons for revision of some previous heat flow calculations. A significant result of revising some of the previous heat flow values is that the high heat flow zone that previously has been shown to extend through North Dakota into Canada is a region of normal continental heat flow.
Age of the Peach Springs Tuff, southeastern California and western Arizona
Authors: Nielson J. E. , Lux D. R. , Dalrymple G. B. , Glazner A. F.
Published: February 2008

Sanidine separates from pumice of the early Miocene Peach Springs Tuff are concordantly dated at 18.5±0.2 Ma by two isotopic techniques. The Peach Springs Tuff is the only known unit that can be correlated between isolated outcrops of Miocene strata from the central Mojave Desert of southeastern California to the western Colorado Plateau in Arizona, across five structural provinces, a distance of 350 km. Thus the age of the Peach Springs Tuff is important to structural and paleogeographic reconstructions of a large region. Biotite and sanidine separates from bulk samples of the Peach Springs Tuff from zones of welding and vapor‐phase alteration have not produced consistent ages by the K‐Ar method. Published ages of mineral separates from 17 localities ranged from 16.2 to 20.5 Ma. Discordant 40Ar/39Ar incremental release spectra were obtained for one biotite and two of the sanidine separates. Ages that correspond to the last gas increments are as old as 27 Ma. The 40Ar/39Ar incremental release determinations on sanidine separated from blocks of Peach Springs Tuff pumice yield ages of 18.3±0.3 and 18.6±0.4 Ma. Laser fusion measurements yield a mean age of 18.51±0.10. The results suggest that sanidine and biotite K‐Ar ages older than about 18.5 Ma are due to inherited Ar from pre‐Tertiary contaminants, which likely were incorporated into the tuff during deposition. Sanidine K‐Ar ages younger than 18 Ma probably indicate incomplete extraction of radiogenic 40Ar, whereas laser fusion dates of biotite and hornblende younger than 18 Ma likely are due to postdepositional alteration. Laser fusion ages as high as 19.01 Ma on biotite grains from pumice suggest that minerals from pre‐Tertiary country rocks also were incorporated in the magma chamber.
Tertiary basin development and tectonic implications, Whipple Detachment System, Colorado River Extensional Corridor, California and Arizona
Authors: Nielson J. E. , Beratan K. K.
Published: February 2008

A stratigraphic and structural record of synchronous extension and detachment faulting is preserved in predominantly Miocene deposits of the Whipple detachment system, Colorado River extensional corridor. In the Mohave Mountains and Aubrey Hills of Arizona and the eastern Whipple Mountains of California near Parker Dam, these deposits comprise four unconformity‐bounded sequences composed of locally derived epiclastic and volcanic rocks and the Peach Springs Tuff. The three older sequences represent syntectonic units that were deposited coeval with detachment faulting, and the fourth is interpreted to be postextensional. The sequences are correlated between four fault‐bounded regions, which are the remnants of four different depositional basins. Similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units. The basins developed in about the same positions, relative to each other and to volcanic sources, as they occupy at present. This is shown by gradational changes of pre‐Tertiary rock types between regions, systematic variations in the abundance of magmatic units, and correlative volcanic units that occur in two adjacent regions. The basins formed in the early Miocene from segmentation of the upper crust into blocks bounded by high‐angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low‐angle detachment fault. Extreme rotation of one large crustal block, which constitutes the central Mohave Mountains, is recorded by a major unconformity in the lower Miocene section of one basin. Because coeval sections that formed in adjoining basins do not record this rotation, the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently. These proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the Whipple detachment system.
Lithology and evolution of the crust‐mantle boundary region in the southwestern Basin and Range Province
Authors: Wilshire H. G.
Published: February 2008

Mantle and crustal xenoliths from volcanic rocks in the southwestern Basin and Range province and Colorado Plateau Transition Zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region. Seismic transects in this area show a strongly reflective Moho of generally low relief, which, in the area of modern transects, consists of a thin zone (<2 km thick) of short reflectors. The upper mantle is transparent and has a Pn of 7.8–8.0 km/s similar to much of the western United States. A lower crustal zone, 2–13 km thick, has variable internal reflectivity and a relatively low velocity of 6.6–6.8 km/s. Upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion; intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths. Whereas melt infiltration preceded and followed ductile deformation, brittle deformation, represented by closely spaced joint systems and faults, followed ductile deformation and is related to the youngest magmatic episodes. These structural characteristics and high uppermost mantle temperature (∼1000°C) may combine to explain the relatively low Pn. Alternating layers of ductily deformed and undeformed peridotites, with or without igneous intrusions, may contribute to the reflectivity of the Moho. Lower crustal xenoliths are dominantly igneous‐textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths. The crustal xenoliths also commonly are jointed, and in addition many show partial melting and have abundant cavities that probably were filled with CO2‐rich fluids. These rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records. The mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems, partial melts, and high temperatures. Garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late Miocene extension. Felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust. The preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes. The earliest magmatic events preceded extension, and later events accompanied and followed extension.
A quantitative study of the axial topography of the Mid‐Atlantic Ridge
Authors: Malinverno Alberto
Published: February 2008

Some basic characteristics of ridge axis topography are related to spreading rate and distance from neighboring transform faults. For example, the presence of an axial depression coincides in most cases with slow spreading rates, and the overall depth of the ridge axis increases toward ridge‐transform intersections (RTIs). On the other hand, it is also well known that the relief and width of the axial valley on, say, the Mid‐Atlantic Ridge (MAR) vary along strike in an unpredictable manner. The purpose of the present study is to quantify how much of the observed variation in the first‐order topography at the axis is related to changes in other parameters, such as spreading rate and distance from RTIs. To carry out this test, the zero‐age depth and the relief and width of the axial valley have been estimated on 46 profiles that cross the axis of the MAR between the equator and 50°N (full spreading rates 22–36 km/Ma). Zero‐age depth is here defined to be the depth at age zero of the best fit thermal subsidence trend. Axial valley relief and width have been measured with respect to the ridge flanks by the least squares fit of a Gaussian bell. The measured axial valley relief varies between 600 and 2100 m (average ∼1300 m), while the valley width varies between 16 and 62 km (average ∼35 km). The correlation between zero‐age depth, axial valley relief and width, latitude of axial crossing, spreading rate, distance from nearest RTT, and offset on the nearby transforms has been investigated using linear regression techniques. The main results of the present study are that (1) zero‐age depth significantly correlates with latitude of crossing, distance from nearest RTI, and offset on the nearby transforms; and (2) the variation in axial valley relief and width is essentially uncorrelated with spreading rate, zero‐age depth, distance from nearest RTT, and offset on nearby transforms. The preferred explanation for the observed spatial variation in axial valley geometry is that it reflects a temporal variation. In fact, if the rough abyssal hill topography typical of the MAR flanks is created within the axial valley, the shape of the axial valley cannot be steady state (although the existence of an axial valley may be a steady state phenomenon). This hypothesis is supported by the observation that the variability in axial valley relief is similar to the overall amplitude of abyssal hill topography, measured as the residual on the thermal subsidence trend.
Cooling rate and thermal structure determined from progressive magnetization of the Dacite Dome at Mount St. Helens, Washington
Authors: Dzurisin Daniel , Denlinger Roger P. , Rosenbaum Joseph G.
Published: February 2008

Our study of a magnetic anomaly associated with the recently active dacite dome at Mount St. Helens suggests that the dome consists of a hot, nonmagnetized core surrounded by a cool, magnetized carapace and flanking talus. The talus does not contribute to the anomaly because its constituent blocks are randomly oriented. Temporal changes in the magnetic anomaly indicate that the magnetized carapace thickened at an average rate of 0.03±0.01 m/d from 1984 to 1986. Petrographic and rock magnetic properties of dome samples indicate that the dominant process responsible for these changes is magnetization of extensively oxidized rock at progressively deeper levels within the dome as the rock cools through its blocking temperature, rather than subsequent changes in magnetization caused by further oxidation. Newly extruded material cools rapidly for a short period as heat is conducted outward in response to convective heat loss from its surface. The cooling rate gradually declines for several weeks, and thereafter the material cools at a relatively constant rate by convective heat loss from its interior along fractures that propagate inward. The rate of internal convective heat loss through fractures varies with rainfall, snowmelt, and large‐scale fracturing during subsequent eruptive episodes. In accordance with a model for solidification of the 1959 lava lake at Kilauea Iki, Hawaii, we picture the dome's magnetized carapace as being a two‐phase, porous, convective zone separated from the nonmagnetized core of the dome by a thin, single‐phase conductive zone. As a consequence of the heat balance between the conductive and convective zones, the blocking‐temperature isotherm migrates inward at a relatively constant rate. If the dome remains inactive, the time scale for its complete magnetization is estimated to be 18–36 years, a forecast which can be refined by shallow drilling into the dome and by continuing studies of its growing magnetic anomaly.
"Biogenic magnetite from surface holocene carbonate sediments,Great Bahama Bank""
Authors: McNeill Donald F.
Published: February 2008

Magnetic separates and rock magnetic tests from Holocene carbonate surface sediments of the Bahamas indicate that fine‐grained magnetite/maghemite is common at or near the sediment/water interface and forms a major portion of the magnetic fraction. Magnetite crystals range from 400 to 1000 Å in diameter as observed under the transmission electron microscope (TEM) and often occur as elongate chains or multi‐grain clusters. Crystals are very similar in dimension and grain habit to those of known biogenic origin, mainly bacteria. All measured grains are within the single‐domain stability field for magnetite based on crystal dimensions. Coercivity distributions are fairly broad (5–100 mT or greater), likely the result of partial oxidation of the magnetic minerals. No large multidomain grains were observed in the TEM study. Individual grains occur in several shapes in the microscope image, mainly hexagonal and cuboidal/prismatic. Widespread occurrence of single‐domain magnetite in this isolated carbonate setting suggests that biogenic precipitation is a major contributor of early in situ depositional magnetization. A contribution of magnetite from authigenic precipitation during burial has not been documented in these near‐surface environments.
Magnetic mineral diagenesis in suboxic sediments at Bettis Site W‐N, NE Pacific Ocean
Authors: Karlin Robert
Published: February 2008

The effects of iron diagenesis on paleomagnetism and rock magnetism were examined in suboxic sediments from Bettis Site W‐N in the NE Pacific Ocean. As part of the oxidative decomposition of organic matter, ultrafine‐grained authigenic magnetites are produced immediately above the iron reduction zone. This zone is commonly observed in deep‐sea sediments as a prominent color change from brown to greenish grey. The magnetite authigenesis is evidenced by systematic increases in natural magnetic intensities and stabilities and by comparable shifts in other rock magnetic properties. The magnetites are composed of relatively pure Fe3O4 and fall within a restricted size range of 0.06–0.12 μm, which is nominally single domain. Some of these finest‐grained magnetites are rapidly destroyed during burial upon entering the zone of iron reduction, leaving a relatively coarser population. Magnetite dissolution appears to continue with depth according to a first‐order surface area reaction process when at steady state. Iron reduction is independent of sulfide formation in these sediments, because solid Sulfur profiles show no increase at depth and most of the sulfur occurs as barite. Despite a well‐constrained carbonate stratigraphy, comparisons of paleomagnetic directions between nearby cores show little agreement, suggesting that the directions are composed of a complex mixture of remanences acquired at different depths. Subsurface magnetic peaks may represent paleoredox markers preserved by non‐steady state redox conditions caused by climatically induced variations in organic matter flux and sedimentation rates.
Remagnetizations and thrusting in the Idaho‐Wyoming Overthrust Belt
Authors: McWhinnie S. T. , Van Der Pluijm Ben A. , Van Der Voo Rob
Published: February 2008

A study on the relationship between deformation and paleomagnetism was carried out in the frontal segment of the Idaho‐Wyoming Overthrust Belt. Samples were taken from the Jurassic Twin Creek unit of the most easterly thrust, the Prospect Thrust, for paleomagnetic study. Detailed analysis of the paleomagnetic data showed a synfolding remagnetization with a paleopole (latitude/longitude: 83°N/286°E) located on the Tertiary segment of the North American apparent polar wander path (APWP). The age of the paleopole roughly coincides with the age of thrusting on the Prospect Thrust as determined by synorogenic deposits. The magnetization was found to reside in magnetite spherules. Previous data from the Absaroka and Darby Thrust sheets that lie to the west of the Prospect Thrust were reevaluated, and they also indicate a synfolding age for the remagnetization. The corresponding paleopole (64°N/162°E) lies on the late Mesozoic segment of the North American APWP. For these more interior thrust sheets, the apparent age of the remagnetization is older than that for the exterior Prospect Thrust sheet, but again roughly corresponds with the inferred age of thrusting for the sampled thrusts. We conclude that remagnetization in the Idaho‐Wyoming Overthrust Belt is temporally related to deformation, and we suggest that authigenic magnetite growth occurred during discrete fluid pulses that were concurrent with thrusting and folding events.
Dynamic elasticity of microbedded and fractured rocks
Authors: Frazer L. Neil
Published: February 2008

Microbedded rocks have an anisotropic frequency‐dependent sound speed which depends on the intrinsic sound speeds of the individual microbeds and on the O'Doherty‐Anstey effect. Fractured rocks have an anisotropic frequency‐dependent sound speed which depends on the intrinsic sound speed of the unfractured rock, the frequency‐dependent phase shift that occurs during reflection or transmission across a fracture, and the interfracture O'Doherty‐Anstey effect. These effects are neglected by the quasistatic methods presently used to generate elastic constants. Here I introduce a new method for generating elastic constants that contain all the above effects. First, a statistical description of the rock is used to generate a sample of the rock. Then an exact two‐way method is used to propagate just a few plane waves, of frequency ƒ, a distance of several wavelengths from the source. If an equivalent homogeneous medium exists at frequency ƒ, then the computed motions must also satisfy a one‐way elastic wave equation for that equivalent medium. This one‐way wave equation is used to invert for the elastic coefficients. When no equivalent medium exists, perhaps because ƒ is too large, this is indicated by the inversion. Possible applications of the method are prediction of seismic sound speeds from measurements of bed thicknesses in cores; analysis of laboratory data for fracture constitutive relations; and inversion of multioffset vertical seismic profiling data for elastic coefficients comparable with those predicted from cores.
Mid‐ocean ridge basalt degassing: Bubble nucleation
Authors: Bottinga Y. , Javoy M.
Published: February 2008

Because of the relatively large surface energy of liquid basalt, the nuclcation of CO2 bubbles in mid‐ocean ridge basalts (MORB) requires degrees of supersaturation, which vary from 1.5 to 7, depending on the pressure. This pressure sensitivity is a direct consequence of the increase in the molar volume of gaseous CO2 when the pressure decreases. This causes the surface area of stable nuclei with a constant number of carbon dioxide molecules to become larger with decreasing pressure. Of the major igneous gases, CO2 is the most abundant in uncontaminated MORB, but has the smallest solubility. Therefore bubbles in MORB at P > 100 bar are CO2 dominated. The recently discovered CO2 supersaturation in MORB [Dixon et al., 1988; Fine and Stolper, 1986] can be explained as being due to the difficulty of bubble nucleation at low pressure.