bluuebunny/crossref_metadata_2025 huggingface dataset:
Magmatic inclusions in silicic and intermediate volcanic rocks
Authors: Bacon Charles R.
Published: February 2008

Fine‐grained ellipsoidal inclusions from a few millimeters to over l m in size are present in many intermediate to silicic lava flows and domes. Only recently has it become widely accepted that such inclusions are chilled blobs of magma. Their magmatic origin is manifested by vesicularity and high groundmass porosity, by ellipsoidal shapes, by mingling at contacts with the host, and by textural evidence for groundmass crystallization in an undercooled state. Groundmass textures reflect the degree of undercooling, which is a function of thermal contrast (ΔT) between inclusions and host magmas before mingling. Compositions of inclusions are invariably more mafic than their hosts and indicate that the magmatic systems were compositionally zoned. Many inclusions were formed from hybrid magmas. Four examples are given in order to document effects of decreasing ΔC (compositional contrast) and ΔT, as hosts for mafic andesitic to andesitic inclusions vary from high‐silica rhyolite (HSR) to silicic andesite. In the HSR example from the Coso volcanic field, California, inclusions track the evolution of zonation in a long‐lived magmatic system. At the other extreme, andesitic inclusions in silicic andesite from Crater Lake, Oregon, demonstrate zonation of a system of low ΔT and ΔC and show effects of in situ differentiation by gas filter‐pressing of residual liquid. Inclusions form when a relatively small proportion of mafic magma comes into contact with a larger proportion of silicic magma. Several mechanisms by which the magmas mix have been proposed: buoyant rise of vesiculated mafic magma or convective stirring at the interface between mafic and overlying silicic magma, forcible injection of mafic magma, diking of fractured silicic magma, and mixing during ascent in a conduit. No single mechanism explains all field observations. Many inclusions in plutons are analogous to magmatic inclusions in volcanic rocks but are likely to have suffered greater chemical and textural change during comparatively slow postentrapment cooling. Magmatic inclusions are evidence of open system behavior of the differentiated parts of magma bodies, and the inclusions themselves may behave as open systems where ΔT is low.
Replenishment of magma chambers by light inputs
Authors: Huppert Herbert E. , Sparks R. Stephen J. , Whitehead John A. , Hallworth Mark A.
Published: February 2008

Magma chambers, particularly those of basaltic composition, are often replenished by an influx of magma whose density is less than that of the resident magma. This paper describes the fundamental fluid mechanics involved in the replenishment by light inputs. If ρ denotes the uniform density of the resident magma and ρ — Δρ that of the input, the situation is described by the reduced gravity g’ = gΔρ/ρ, the volume flux Q, and the viscosities of the resident and input magmas νe and νi, respectively. The (nondimensional) Reynolds numbers, Ree = (g’Q3)1/5/νe and Rei = (g’Q3)1/5/νi and chamber geometry then completely specify the system. For sufficiently low values of the two Reynolds numbers (each less than approximately 10), the input rises as a laminar conduit. For larger values of the Reynolds numbers, the conduit may break down and exhibit either a varicose or a meander instability and entrain some resident magma. At still larger Reynolds numbers, the flow will become quite unsteady and finally turbulent. The values of the Reynolds numbers at which these transitions occur have been documented by a series of experiments with water, glycerine, and corn syrup. If the input rises as a turbulent plume, significant entrainment of the resident magma can take place. The final spatial distribution of the mixed magma depends on the geometry of the chamber. If the chamber is much wider than it is high, the mixed magma forms a compositionally stratified region between the roof and a sharp front above uncontaminated magma. In the other geometrical extreme, the input magma is mixed with almost all of the resident magma. If the density of the resident magma is already stratified, the input plume may penetrate only part way into the chamber, even though its initial density is less than that of the lowest density resident magma. The plume will then intrude horizontally and form a hybrid layer at an intermediate depth. This provides a mechanism for preventing even primitive basaltic magmas of minimum density from erupting at the surface. By conducting an experiment using aqueous solutions, we show that entrainment can lead to crystallization of the magma in the input plume by making it locally supersaturated. All these effects are discussed and illustrated by photographs of laboratory experiments.
Jemez Mountains volcanic field, New Mexico: Time term interpretation of the CARDEX seismic experiment and comparison with Bouguer gravity
Authors: Olsen K. H. , Braile L. W. , Stewart J. N. , Daudt C. R. , Keller G. R. , Ankeny L. A. , Wolff J. J.
Published: February 2008

The Jemez Mountains volcanic field (JVF) is centered astride the western flank of the Rio Grande rift in north central New Mexico. Near the center of the volcanic field are the Valles and Toledo calderas, the sites of two cataclysmic eruptions of silicic tephra and subsequent caldera collapses. These calderas are among the largest and most recent (1.1 and 1.4 Ma) of the world's giant volcanic calderas. The Valles caldera, especially, has associated highly developed hydrothermal systems and may still contain residual magma within a cooling, compositionally zoned pluton at shallow crustal depths. The main goal of our study was to estimate the complex shallow crustal structure beneath the Jemez volcanic field using data from a seismic refraction experiment that encompassed the central part of the field. A time term technique was used to process first arrival time data from the network of temporary seismic stations of the Caldera and Rift Deep Seismic Experiment (CARDEX) refraction experiment. The data set, consisting of 290 time‐distance pairs, was processed to yield time terms at 84 locations (78 recording stations and 6 explosion sites). The P wave velocity of the Precambrian basement refractor was estimated to be 5.86 ± 0.01 km/s. Station time terms are highly correlated to Bouguer gravity anomalies and thus tend to corroborate models for which the crystalline basement outcrops along the western margins of the rift and western flank of the JVF and dips generally eastward to depths of at least 600 m below sea level beneath sediment‐filled axial basins of the rift.
Estimates of terrestrial heat flow from deep petroleum tests along the Rio Grande Rift in central and southern New Mexico
Authors: Reiter Marshall , Eggleston Roberta Eaton , Broadwell Brenda R. , Minier Jeffrie
Published: February 2008

Twenty‐six new estimates of heat flow are calculated for the Albuquerque‐Belen Basin and the basins of the southern Rio Grande rift in New Mexico. From these and previous data the following heat flow patterns may be suggested. The regional or “background” heat flow in the Albuquerque‐Belen Basin is ∼77 mw m−2. There is also a NE‐SW trend of higher heat flow (92–110 mW m−2) across the region. In the southern Rio Grande rift, between Socorro and Las Cruces, the regional heat flow appears to be ∼95 mW m−2. In this region there are also higher heat flows (117–135 mW m−2) at widely scattered sites. South of Las Cruces, near the international border between Mexico and the United States, preliminary and perhaps suspect data suggest the possibility of a transition to the intermediate heat flows previously estimated at sites throughout much of northeastern Chihuahua, Mexico. Regional heat flow values in the Albuquerque‐Belen Basin, and in the basins of the southern Rio Grande rift, are consistent with, but do not constrain, simple models of crustal thinning. Higher heat flows in these two areas are thought to be caused by hydrothermal fluids (largely groundwaters but possibly magma in some areas) transporting heat upward along crustal fracture zones.
On the existence of a periodic dislocation cycle in horizontally layered viscoelastic models
Authors: Bonafede Maurizio , Dragoni Michele , Morelli Andrea
Published: February 2008

Horizontally stratified viscoelastic models are employed to characterize the lithosphere‐asthenosphere interaction following dislocation events occurring on long transcurrent faults. The aim is to investigate the ability of such models to reproduce the periodic behavior often assumed in the geophysical literature for the earthquake source process. If the lithosphere is elastic and the asthenosphere has a Maxwell rheology, the earthquake recurrence time is proved to become zero (model I) or infinity (model II) if boundary conditions of constant strain rate or limited tectonic stress, respectively, are imposed over the fault region. Model assumptions that may be responsible for these paradoxical results include the choice of boundary conditions, the half‐space two‐dimensional geometry, and the assumed rheological behavior. This third possibility has been investigated in some detail. The adoption of the standard linear solid for the asthenosphere (model III) yields a stationary nonvanishing recurrence time if constant strain rate is applied. Another possibility is to assign a viscoelastic rheology to the lithosphere also (model IV); in this case, an earthquake cycle is obtained whose period depends on the assigned boundary conditions. In any case, the results obtained clearly indicate the importance of past earthquakes in the earthquake cycle.
Formation of seafloor swells by mantle plumes
Authors: Olson Peter , Nam In Seung
Published: February 2008

The prominent oceanic hot spots are located at the crest of broad topographic swells on the seafloor. Available evidence indicates that swells are supported by anomalously thin, or altered, lithosphere as might result from the intrusion of a mantle plume into the lithosphere. A set of experiments have been made to measure the free surface uplift and subsidence produced by intrusion of a low‐viscosity, buoyant plume into a rheological boundary layer. A deep layer of strongly thermoviscous sugar solution with a free surface was cooled from above to simulate the lithosphere‐asthenosphere system. Low‐viscosity, low‐density plumes were released at depth. The free surface displacement generated by the ascending plume was recorded as a function of time using a proximity measurement system. The plumes ascend through the asthenosphere as nearly spherical diapirs and collapse (flatten) upon intrusion into the base of the lithosphere. Evolution of the surface topography can be divided into three phases: (1) a low axisymmetric rise supported by the plume deep beneath the lithosphere, (2) rapid uplift and steepening as the plume reaches the lithosphere base, and (3) slow axial subsidence as the plume stagnates within the lithosphere. The measured uplifts are compared with the uplift calculated for buoyant plumes in a uniform viscous half‐space, using Morgan's image method. The agreement is good for thin lithosphere. For thick lithosphere the observed is less than the calculated uplift. Scaling laws have been derived which relate the measured maximum uplift and maximum uplift rate to diapir parameters. Extrapolation of these to the scale of ocean floor swells gives uplift rates of approximately 1 km in 5 m.y. by this mechanism. The most significant qualitative results of the experiments are that swells formed by interaction of low‐viscosity mantle plumes and oceanic lithosphere can be characterized by axial symmetry, rapid uplift, and shallow compensation.
Exact and numerical solutions to the moving boundary problem resulting from reversible heterogeneous reactions and aqueous diffusion in a porous medium
Authors: Lichtner Peter C. , Oelkers Eric H. , Helgeson Harold C.
Published: February 2008

Numerical finite difference calculations for diffusional mass transport incorporating reactions between minerals and a fluid phase, based on a continuum representation of porous media, are compared with the exact solution. The finite difference algorithm is based on the weak formulation of the moving boundary problem in which a fixed grid of node points is used. Mineral reactions are considered to be in local equilibrium with a fluid phase and may take place either at sharp reaction fronts or distributed homogeneously throughout a control volume. The theory is applied to one‐ and two‐component systems. Analytical solutions to the finite difference equations for the first and second node points provide for a detailed comparison with the exact solution. It was found that on a time scale that is small compared with the time required for the solid to completely dissolve at a single node point, the finite difference approximation yields a spurious behavior for the concentration and solid phase volume fraction. However, the finite difference algorithm reproduces the average behavior of the motion of the reaction front and concentration of the reacting species provided the advance of the front is sufficiently slow resulting in a quasi‐steady state solution. A numerical example is presented for the dissolution of quartz at 550°C and 1000 bars to illustrate the general theory.
Rheology of synthetic olivine aggregates: Influence of grain size and water
Authors: Karato Shun‐Ichiro , Paterson Mervyn S. , FitzGerald John D.
Published: February 2008

The influence of grain size and water content on the high‐temperature plasticity of olivine aggregates was studied, using a gas‐medium high‐pressure deformation apparatus. The specimens used were hot‐pressed, dense olivine aggregates with controlled grain size ranging from a few to 70 μm, with or without added water. Mechanical tests were made at 1573 K and 300 MPa confining pressure and at strain rates of 10−3 to 10−6 s−1. The results reveal two distinct mechanisms of deformation, depending on stress level and grain size. At relatively high stress and large grain size, the strain rate is proportional to about the cube power of the stress and is nearly independent of grain size. In this regime, microstructural observations gave evidence of intragranular deformation involving dislocation motion. At low stress and small grain size, the strain rate depends almost linearly on stress and decreases markedly with increase in grain size. In the latter regime, little evidence was found for intragranular deformation. These observations suggest that the deformation mechanism in the grain size insensitive regime is dislocation creep, while that in the grain size sensitive regime is diffusion creep. In both regimes, water was found to enhance the creep rate. The absence of grain size sensitivity in the dislocation creep regime and comparison with single‐crystal data indicate that the water weakening effect is mainly an intragranular process. However, the existence also of a water weakening effect in the diffusion creep regime indicates that water also enhances diffusion. The extrapolation of the present results to coarser grain sizes indicates that the transition from dislocation to diffusion creep occurs at 0.1–1 MPa for 10‐mm grain size. Therefore it is suggested that this transition may occur in the upper mantle and that, in both regimes, the presence of trace amounts of water will result in significantly lower creep strength than under strictly “dry” conditions.
Paleomagnetism of Middle Tertiary volcanic rocks from the Western Cascade Series, northern California
Authors: Beck Myrl E. , Burmester Russell F. , Craig Douglas E. , Gromme' C. Sherman , Wells Ray E.
Published: February 2008

The Western Cascade Series (WCS) is a 3.5‐km‐thick, crudely homoclinal (east dipping) calcalkaline volcanic sequence of mid‐Oligocene to early Miocene age that crops out near the southern tip of the Cascade Range in northern California. The mean direction of remanent magnetization in the WCS is D, 4.9°; I, 57.6° (N, 53; k, 14.4; α95, 5.3°). When compared to a reference direction for the North American craton, the WCS direction indicates that the southern Cascade Range has rotated 14.0° ± 9.0° since the WCS accumulated. A difference in mean direction between the lower and upper halves of the WCS suggests that much of this rotation occurred during the late Oligocene. Six other paleomagnetic studies of rock units of an age roughly comparable to the WCS also are available from western Oregon, northwestern California, and southwestern Washington. All show statistically significant clockwise rotation (inclinations of all but one are concordant). Comparison of directions indicates that the Pacific Northwest did not rotate as a rigid body, but neither did it behave as a collection of small, independently rotating domains. The amount of rotation found throughout the area increases to the west or northwest, suggesting a driving force for rotation operating at the continental margin.
Resistivity structure to a depth of 5 km beneath Kilauea Volcano, Hawaii from large‐loop‐source electromagnetic measurements (0.04–8 Hz)
Authors: Kauahikaua Jim , Jackson Dallas B. , Zablocki Charles J.
Published: February 2008

A deep resistivity study of Kilauea volcano using the controlled‐source electromagnetic (CSEM) technique has revealed that the summit region (i.e., the area bounded by the east rift zone, the southwest rift zone, and the Koa'e fault zone) consists of subhorizontal layers having resistivities ranging from 2 ohm‐m to greater than several hundred ohm meters. The vertical sequence of resistivities is consistent with an increase in temperature from 20° C at the surface to over 1000° C at a depth of about 5 km. Temperatures beneath the adjacent but inactive area of Mauna Loa probably do not exceed 300° C at a comparable depth. A deep conductor, which is indicated by data at five observation locations on Kilauea, may be magma. The source for the CSEM study is a horizontal wire square approximately 2 km on a side, through which a sinusoidal current of about 80 A (peak‐to‐peak) is driven at frequencies between 0.04 and 8 Hz.
Seasat‐derived gravity over the Musicians Seamounts
Authors: Freedman Adam P. , Parsons Barry
Published: February 2008

The two‐dimensional gravity field over the Musicians seamount province in the Pacific Ocean has been derived from Seasat altimetry. Geoid maps were produced by fitting a minimum curvature surface to the sea surface height measurements. As a check on the quality of this interpolation method, we also gridded the data using weighted grid point averages. Fourier transforms of both the geoid and geoid gradient were used to determine the gravity field. We have compared gravity maps produced these different ways in order to identify the problems involved in pushing Seasat data to the limits of its spatial resolution and accuracy. Our minimum‐curvature interpolation scheme was determined to be the more accurate and cost effective mapping method, while gravity obtained by transforming the geoid produced more reliable gravity maps. The bathymetry of this area was used to predict the gravity field by filtering the bathymetry under the assumption that it is regionally compensated by a thin elastic plate. Gravity fields predicted for a variety of effective elastic thicknesses were compared to the Seasat‐derived gravity, particularly in areas with good track coverage. The derived gravity tends to favor a thin plate with an effective elastic thickness of ∼5 km, though the east‐west ridges in the south display a smaller signal more consistent with Airy compensation. This variation may be indicative of early fracturing of the lithosphere in the south, or it may be a manifestation of the age difference and early thermal structure across the Murray fracture zone, which separates the seamount province into northern and southern sections. Neighboring seamounts with different flexural signatures, particularly in the south, may indicate that volcanism occurred in the same location over an extended period of time.
Precision of vertical position estimates from Very Long Baseline Interferometry
Authors: Herring T. A.
Published: February 2008

We discuss the prospects for using very long baseline interferometry (VLBI) to measure vertical crustal motions. Our analysis of this problem indicates that the main limitations of such VLBI measurements will probably arise from errors in modeling of the propagation delay through the earth's neutral atmosphere and errors in determining the orientation of a “crust fixed” coordinate system in the VLBI (inertial) reference frame. We examine two techniques for assessing the precision of vertical position measurements which can be made with currently available VLBI systems: (1) the repeatability of baseline length measurements, and (2) the repeatability of the vertical site position of one site in a four‐site network when the positions of the other three sites determine the earth orientation parameters. The repeatability of baseline length measurements implies that the precision of the vertical position estimates is ∼8 cm, averaged over 13 sites (separated by 600–8000 km) and 4.5 years of data. The repeatability of vertical position estimates for the Richmond, Florida, site is ∼7 cm for the 42 observing sessions carried out during an 11‐month period. Both of the techniques we have used to estimate the precision of height determinations indicate that the current precision is ∼8 cm for a single 24‐hour VLBI observing session. The effects of errors in the earth orientation parameters will depend on the distances between the VLBI sites and could induce errors as large as the precision given above.
Ascent of magma in a deformable vent
Authors: Ida Yoshiaki , Kumazawa Mineo
Published: February 2008

The ascent of magma in a deformable cylindrical vent is analyzed to reveal the character and extent of mechanical disturbances in vent geometry and associated effects on the magma ascent process. Both magma and surrounding country rock are assumed to behave as Newtonian fluids. According to the analysis, any disturbance involving the deformation of conduit is transmitted upward at approximately same speed as magma motion, while the disturbance hardly influences the state below. In particular, irregular vent geometry containing a bump or a constriction propagates upward like a wave, being accompanied by a pressure dipole that is composed of a pair of positive and negative pressure deviations. Since magma flux is strongly controlled by the pressure below, the persistence of high magma pressure in the magma reservoir favors expansion of the vent and can lead to a volcanic eruption. On the contrary, a low magma pressure tends to result in the closure of the conduit above. Therefore if the effective viscosity of the country rock is sufficiently low as might be the case when it is partially molten, a vent can be flexibly opened or closed in response to pressure in the magma reservoir.
Viscosity of partial melts in the upper mantle
Authors: Kushiro Ikuo
Published: February 2008

The viscosity and density of melts of composition close to primary olivine tholeiite and alkali olivine basalt have been measured at pressures between 0.7 and 1.5 GPa and at temperatures between 1300° and 1475°C using the falling/floating sphere method. The viscosity of the olivine tholeiite melts decreases only slightly from 1 atm to 1.0 GPa, whereas that of the alkali olivine basalt melts decreases by a factor of about 2 in the same pressure range, and they become very close to one another (about 2.0 Pa s) at 1.0–1.25 GPa at 1400°C. The activation energy for viscous flow of the olivine tholeiite melt is about 250 kJ/mol at 1.0 GPa. The viscosity of partial melts (melts formed by partial melting) in the upper mantle can be estimated based on the viscosities of the melts and the activation energy for viscous flow obtained by the present experiments. The viscosity of the partial melts along the anhydrous solidus of peridotite decreases from about 2.5 Pa s at 0.8 GPa (1220°C) to only about 0.2 Pa s at 3.5 GPa (1580°C). Thus, the partial melts must be extremely fluid at depths greater than 100 km even under anhydrous conditions. Such partial melts might not ascend because of their high density and play an important role in the movement of the lithosphere. The viscosities of the partial melts in the upper mantle may decrease with increasing depth until completion of the transformation of Si to six‐fold coordination. The viscosities of magmas and the diffusivity of oxygen in them have an inverse relation, which is approximated by the Eyring's equation. The diffusivity of oxygen in the partial melts, estimated from this relation and the viscosity, increases from 10−7 cm2/s at 0.8 GPa to 10−5 cm2/s at 3.5 GPa along the anhydrous solidus of peridotite. Homogenization of partial melts or magmas would be greatly enhanced with increasing depth down to the viscosity minimum.
Hysteresis properties of magnetite and their dependence on particle size: A test of pseudo‐single‐domain remanence models
Authors: Dunlop David J.
Published: February 2008

This paper reports room temperature measurements of initial susceptibility χ0, second Rayleigh coefficient B, coercive force Hc, remanent coercive force HR, and the ratio Jrs/Js of near‐saturation remanent and induced magnetizations for four samples containing single‐crystal magnetite cubes with mean sizes of 0.037, 0.076, 0.10, and 0.22 μm. While χo and Hc have weak dependences on particle size d, Hc and Jrs vary strongly with size, as d−0.55 and d−0.75, respectively, over the 0.076–0.22 μm range. These hysteresis properties and their size dependences are incompatible with data for >l‐μm crushed and annealed magnetite grains. Nearly perfect unstrained crystals tend to have Hc and Jrs values 2–5 times smaller than those of pulverized grains of nominally the same size. Magnitudes and size dependences of Hc and Jrs for >0.06‐μm magnetites are approximately, although not perfectly, accounted for by (1) metastable single‐domain (SD) particles, (2) rotatable SD‐like domain wall (DW) moments, or (3) DW displacements impeded by crystal defects and limited by self‐demagnetization. Only reversible DW displacements (or possibly domain rotations) opposed by self−demagnetization can explain the initial susceptibility data, however.
Dynamics of sediment subduction, melange formation, and prism accretion
Authors: Shreve Ronald L. , Cloos Mark
Published: February 2008

The descending plate and overriding block in a subduction zone are analogous to the guide surface and slide block in a slipper bearing, and subducted sediment is analogous to the lubricant. Subduction is more complex and varied, however, because the overriding block is not rigid, the sediment is buoyant, underplating can occur, and sediment supply can vary widely. A model based on the bearing analogy but taking these differences into account makes detailed quantitative predictions for actual sites, which are illustrated by calculations for five diverse examples: Mariana, 16°N; Mexico, 17°N; Lesser Antilles, 13°N (Barbados); Alaska, 153°W (Kodiak); and Japan, 40°N. It requires as input the geometry of the overriding block and the top of the descending plate, the distribution of density and permeability of the overriding block, the speed of subduction, the density and rheological properties of the subducted sediment, and the rate of sediment input. Its predictions include the profile of thickness of the layer of subducted sediment (all sites; maximum of 360 m at Mariana, 5300 m at Japan), the velocities of flow in the layer (all sites), the shear stresses exerted on the walls (all sites; low beneath accretionary prisms, up to 6 MPa beneath Japan), the rate of offscraping (none at Mariana and late Tertiary Mexico; 85% of input at Lesser Antilles; includes melange at Japan), the distribution and rates of underplating (none at Mariana, extensive at Japan), the zones of possible subduction erosion (extensive at Mariana; local at the others), the amount of sediment subducted to the volcanic arc (all sites; 2% of input at Lesser Antilles, 100% at Mariana), the qualitative pattern of flow at the inlet (five basic patterns; all sites), the upward flow of melange in many instances (none at Mariana; extensive at Japan), and, under relatively rare conditions, the formation of large‐scale melange diapirs (only at Lesser Antilles beneath Barbados Island).
Stratigraphy, paleomagnetism, and magnetic fabric of the Toba Tuffs: Constraints on the sources and eruptive styles
Authors: Knight Michael D. , Walker George P. L. , Ellwood Brooks B. , Diehl Jimmy F.
Published: February 2008

The Toba depression in north central Sumatra is a complex of several overlapping calderas resulting from three major ignimbrite‐forming eruptions. Within the depression, the upland masses of Samosir and northern Uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment, hitherto interpreted to be two parts of a single resurgent dome. This study has demonstrated that the welded tuffs of Samosir and Uluan have different magnetic polarities and therefore at least two different ignimbrites are present; the Samosir/Uluan massif may consist of parts of two resurgent domes. The first ignimbrite eruption occurred at 0.84 Ma and produced a very thick (>400 m), densely welded unit having a reversed polarity. Anisotropy of magnetic susceptibility (AMS) flow direction and lithic size data indicate that the source lies in the southern part of the Toba depression, and the thick deposit of Uluan is thought to have ponded in a 40‐km‐wide caldera. The second ignimbrite is normally magnetized. AMS flow direction data indicate two separate source vents, one to the north in the Haranggaol caldera, and another to the south. The thick deposit at Samosir is thought to have ponded in the southern caldera. Coarse sediments then accumulated over Samosir and northern Uluan and were capped by lacustrine deposits. A renewed episode of resurgence then uplifted Samosir Island and possibly the northern part of Uluan. At approximately 0.075 Ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the Toba depression. This ignimbrite is mostly nonwelded and normally magnetized. Part of the Uluan dome was destroyed by collapse of the Sibandung caldera and Latung graben and concomitant with renewed subsidence of the Haranggaol and Porsea calderas.
Consortium for continental reflection profiling Michigan surveys: Reprocessing and results
Authors: Zhu Tianfei , Brown Larry D.
Published: February 2008

Reprocessing of Consortium for Continental Reflection Profiling data from central Michigan, with emphasis on deconvolution and velocity filtering before stack and more detailed velocity analysis, has resulted in greatly improved imaging of a previously reported Keweenawan rift buried beneath the Paleozoic of the Michigan Basin. The seismic stratigraphy of the sub‐Paleozoic units can now be related to the basin‐filling Middle Keweenawan volcanic sequence and the Upper Keweenawan clastic assemblage in greater detail. Important structural and stratigraphic features can be seen for the first time. For example, reprocessing has imaged an unconformity within the Upper Keweenawan. It is possible some of these newly imaged features are associated with a late Keweenawan compressive episode. Furthermore, reprocessing also reveals previously unrecognized sub‐Keweenawan structures, including a deeply penetrating reflection which may represent a major crustal fault, and possible weak reflections from midcrustal (25 km) and Moho (43 km) depths.
Mass estimates of cumulates and residues after anatexis in the Oslo graben
Authors: Neumann Else‐Ragnhild , Pallesen Stig , Andresen Pål
Published: February 2008

The magmatic province of the Permian Oslo Rift, southeast Norway, is dominated by rocks of monzonitic to granitic composition. Most of the monzonitic rocks are believed to have formed by fractional crystallization from mantle‐derived magmas, whereas melts produced by anatexis in the lower crust are important to the formation of the syenitic and granitic rocks. These processes have left dense cumulates and dense residues after melting in the crust. The surplus masses of cumulates and residues at depth in the crust can be estimated by using petrological and geological information on the rocks exposed today. Quantitative estimates for the Oslo Graben are comparable to corresponding data derived from geophysical crustal models. The petrological/geological estimates suggest excess masses of 7–10×1010 kg per linear meter (assuming prismatic bodies) along the rift axis in the southern, and 5–7×1010 kg/m in the northern part of the Oslo Region. These estimates are dependent upon the volume proportion of trapped liquid in the cumulates, and the ratio of cumulates to residues after anatectic melting in the crust; the amount of trapped liquid appears to be low (less than 25%). The results agree well with estimates by Wessel and Husebye (1986) of excess mass in the crust based on gravimetric data, which gave 5–7×1010 kg/m. This agreement testifies to the validity of making mass estimates on the basis of petrological and geological data. The data bear on the composition and evolution of the upper lithosphere in the graben area; some cumulates may reside in the subgraben mantle.
Genetic environments of the banded sulfur sediments at the Tateyama Volcano, Japan
Authors: Kusakabe Minoru , Hayashi Naomi , Kobayashi Takehiko
Published: February 2008

Banded sulfur sediments, characterized by alternate layers of yellowish, fine‐grained sulfur and dark gray clays, occur in a now‐extinct crater lake of the Tateyama volcano, central Japan. Stratigraphic variations in the sulfur content, sulfur isotopic ratio, mineralogy, and content of diatom fossils suggest that the lamination represents seasonal repetition of environmental conditions of the lake water; colloidal sulfur, formed by oxidation of volcanic H2S in the presence of dissolved oxygen, and freshwater benthonic diatoms (Pinnularia braunii (Grun.) Cl. var. amphicephala (A. Mayer) Hust.), flourished during oxidative‐convective periods, whereas detrital materials rich in clays were deposited onto the lake bottom during reducing‐stagnant periods. A sedimentation period of 3000 to 4000 years is calculated from the total thickness of the banded sulfur sediments coupled with the sedimentation rates of 1 to 2 mm/yr deduced from the above model. The estimated period is in good agreement with the age of phreatic explosion (3000 to 6300 years ago based on tephra chronology), which created the crater lake.
Temperature increase and chemical change of fumarolic gases at Momotombo Volcano, Nicaragua, in 1982–1985: Are these indicators of a possible eruption?
Authors: Menyailov I. A. , Nikitina L. P. , Shapar V. N. , Pilipenko V. P.
Published: February 2008

Active volcanoes of Nicaragua, including Momotombo volcano, were monitored in 1982–1985 to determine volcanic hazard and to establish criteria for prediction of eruptions. In 1982 the chemical composition and isotopic ratios of fumarolic gases were found to depend on temperature. In the products of fumarolic activity the materials of crustal origin are of great significance. Sharp variations in the gas composition depending on exogenic factors were noted in 1983; the H2S/SO2 ratio was influenced the least. The temperature of fumarolic gases at Momotombo gradually increased from 740°C in 1974 to 895°C in 1984. During the same period the contents of H2S, SO2, CO, and H2 and the values of S/Cl, S/C, H2S/SO2, H2/H2O, and CO/CO2 ratios have risen, and the isotope ratios of hydrogen, sulfur, and especially carbon have changed. Future work in the crater of Momotombo will show whether the changes in the temperature and composition of the fumarolic gases are the precursors to a forthcoming eruption.
Factors affecting sulfur gas anomalies in overburden
Authors: Kesler S. E. , Gardner M.
Published: February 2008

Synthetic sulfide ore deposits constructed and tested in our laboratory confirm that oxidizing sulfide deposits generate detectable amounts of COS and CS2. Most of the COS in these experiments was adsorbed onto the soil overlying the sulfide material, indicating that surveys designed to measure COS variations should make use of gases desorbed from soil. In contrast, most of the CS2 in the experiments was found in the head space air, indicating that it would be most easily detected in surveys collecting and analyzing interstitial soil gas. The lack of CS2 adsorbed onto soil is probably a result of its oxidation to COS after being adsorbed onto soil during our experiments.
Paleomagnetic directional dispersion produced by plastic deformation in a thick miocene welded tuff, southern Nevada: Implications for welding temperatures
Authors: Rosenbaum J. G.
Published: February 2008

Two voluminous rhyolitic to quartz‐latitic ash flow sheets, the Tiva Canyon and Topopah Spring members of the Paintbrush Tuff, were erupted from the Miocene Claim Canyon cauldron in southern Nevada. Although lithologically similar, these units differ greatly in their recording of the ancient geomagnetic field. The reversely magnetized Tiva Canyon Member yields remanent directions that are well grouped both within and between sites. The normally magnetized Topopah Spring Member, however, yields remanent directions that vary both laterally and vertically. Remanent directions from a 100‐m‐thick section of nonwelded to partially welded Topopah Spring are well grouped (precision parameter, k > 200) and give a mean direction (D = 293°, I = 69°) that is probably a good approximation of the geomagnetic field at the time of emplacement and cooling. In three 300‐m‐thick sections of more densely welded tuff, inclinations from the upper parts of the Topopah Spring are steep (70°), but flatten with depth to about 20°, and then steepen abruptly near the base of the unit. In general, the directional dispersion increases with the thickness of the section (k ∼ 30 for sections that are about 300 m thick). Much of the directional variation in the Topopah Spring can be attributed to an inclination error produced by subblocking temperature rotation of the magnetic carriers during welding. The possibility of subblocking temperature deformation in an ash flow sheet is enhanced by (1) high‐blocking temperature spectra of the primary magnetic phases, (2) relatively low emplacement temperatures, and (3) great stratigraphic thickness. Thermal demagnetization results from the Topopah Spring Member indicate that welding may have continued to temperatures as low as 475°C in the basal vitrophyre of the ash flow sheet. Remanent directions from the Tiva Canyon Member were unaffected by the welding process because this unit possesses significantly lower blocking temperatures and does not attain the great thicknesses of the Topopah Spring.
Gravity tide measurements with a feedback gravity meter
Authors: Levine Judah , Harrison J. C. , Dewhurst Warren
Published: February 2008

We have analyzed gravity tide data obtained using a calibrated gravity meter with electrostatic feedback. We find good agreement between the measured amplitude and phase of the major semidiurnal components and the corresponding values to be expected using current earth models and ocean load calculations. Both local and global barometric pressure changes make significant contributions to the power in the tidal bands and are included in the fitting function. The admittance estimates at diurnal frequencies can be used to determine the frequency and to set a lower bound on the dissipation of the nearly diurnal resonance in the tidal response. These estimates are in reasonable agreement with results obtained by other methods but are somewhat different than the values to be expected on the basis of theoretical considerations.
Buckminsterfullerene, its siblings, and soot: Carriers of trapped inert gases in meteorites?
Authors: Heymann D.
Published: February 2008

Three‐dimensional, spheroidal structures of carbon atoms, Cn (n > 40), can be made with 12 pentagons and (n/2)−10 hexagons. Kroto et al. (1985) have shown that the molecule C60, called Buckminsterfullerene, is especially stable and that soot might be made of nuclei of Cn shells. Inert gas atoms might become trapped in such structures when they form by the condensation of carbon from a gas phase. Niemeyer and Marti (1981) have shown that trapping in condensing carbon occurs and is attended by mass fractionation. The hypothesis of gas trapping in Cn‐shells and/or in soot particles derived from them might explain the thermal release patterns of trapped inert gases in acid‐resistant residues of carbonaceous chondrites up to at least 1000°C. The hypothesis predicts “graphitization” of the acid‐resistant residues with the gas‐release.
Volcanic Episodicity and a Non‐Steady State Rift Valley Along Northeast Pacific Spreading Centers: Evidence From Sea MARC I
Authors: Kappel Ellen S. , Ryan William B. F.
Published: February 2008

Sea MARC I side‐looking sonar images and Sea Beam bathymetry along a 400‐km stretch of the Juan de Fuca Ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading. An elongate summit depression (ESD), or rift valley, is commonly observed in the spine of this crestal ridge. The crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis. The landform has a plan‐form shape that resembles a side view of an archer's bow with the long dimension of the bow form parallel to the strike of the ridge. In cross section, the bow form is flat on top and has steep flanks. These bow‐form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge‐parallel dikes, similar to Icelandic fissure eruptions. Both the variable dimensions of the ESD along axis of the Juan de Fuca Ridge and the relationship among volcanic flow morphologies within and beyond the ESD suggest the four different segments of the Juan de Fuca Ridge presented in detail here display different stages in a cycle of oceanic crust accretion. This cycle includes episodes in which there is (1) extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression, (2) collapse within the summit region of the crestal ridge to form an ESD during a phase of volcanic inactivity, and (3) renewed magmatism in the ESD as its floor widens by extension and brittle fracture of the upper crust. This episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time.
Distinctive Triassic megaporphyritic monzogranite: Evidence for only 160 km offset along the San Andreas Fault, southern California
Authors: Frizzell Virgil A. , Mattinson James M. , Matti Jonathan C.
Published: February 2008

Distinctive megaporphyritic bodies of monzogranite to quartz monzonite that occur in the Mill Creek region of the San Bernardino Mountains and across the San Andreas fault on Liebre Mountain share identical modal and chemical compositions, intrusive ages, and petrogenesis and similar thermal histories. Both bodies are strontium‐rich and contain large potassium feldspar phenocrysts and hornblende. U‐Pb determinations on zircon from both bodies indicate Triassic intrusive ages (215 Ma) and derivation, in part, from homogeneous Precambrian continental crust. U‐Pb analyses on apatite and sphene and K‐Ar analyses on hornblende and biotite show that the bodies suffered a Late Cretaceous thermal event (70–75 Ma). The strong similarities between the two bodies suggest that they constitute segments of a formerly continuous pluton that has been offset about 160 km by movement on the San Andreas fault, about 80 km less than the generally accepted distance. Plutons having monzonitic compositions, reassembled with the megaporphyritic bodies are used as a piercing point, form a relatively coherent province within the varied suite of Mesozoic batholithic and prebatholithic rocks in southern California.
An experimental study of oxygen transport in dry rocks and related kinetic phenomena
Authors: Watson E. Bruce
Published: February 2008

Several series of experiments were carried out at 1 GPa and 900°–1100° in which grain boundary chemical diffusion of oxygen in dry quartzite and dunite was characterized by use of “indicator mineral” techniques. Most experiments involved mixing a small amount of Fe2O3 with coarse quartz or olivine powder, placing the mixture in an iron container, pressurizing in a piston‐cylinder apparatus, and heating rapidly to the desired run temperature. In response to the large gradient in oxygen fugacity resulting from the juxtaposition of iron metal (i.e., the container) with Fe2O3 disseminated through the sample, oxygen transport along quartz or olivine grain boundaries took place, causing reduction of ferric iron in regions of the sample nearest the container (producing wüstite in the dunite experiments and fayalite in those on quartz). The flux of oxygen, and hence the diffusivity, could be calculated from the width of the observed reduction zone. Variations on the method just described included use of CuO rather than Fe2O3 as the “indicator mineral,” use of graphite rather than iron as the container, and placement of the Fe2O3 as a single pellet at the center of the dunite or quartzite sample rather than as a dispersed minor phase. For the dispersed‐Fe2O3 experiments on dunite at 1000° and 1100°C, the width of the reduced (wüstite‐bearing) zone was found to be generally proportional to the square root of run duration, in accordance with a diffusion‐controlled transport/reduction process. At 900°, however, this relation does not hold, and the data suggest vapor phase transport of oxygen prior to closure of sample porosity. All experiments on quartzite (900°–1100°) were plagued by the same problem, but in this case, sufficiently long experiment durations eventually resulted in no further measurable oxygen transport, demonstrating that well‐annealed quartzite is impermeable to oxygen at the run conditions. The net outcome of the experiments is a bulk grain boundary diffusivity for oxygen in dunite given approximately by D = 106exp (−355,500/RT) (D in cm2/s; R in J/deg mol). Values for quartzite are too low to measure by the techniques attempted. The information can be used to argue that heterogeneities in oxygen fugacity on the scale of centimeters to meters can be maintained indefinitely in fluid‐absent, high‐grade rocks of the continental crust, but similar features in a fluid‐absent mantle would be quickly erased.