Gas Exsolution Research Articles

Volcanogenic upwelling of anoxic, nutrient-rich water: A possible factor in carbonate-bank/reef demise and benthic faunal extinctions?

The middle Cretaceous oceans were characterized by widespread anoxia, high sea levels, reef/carbonate-bank extinctions, and, at least in the Pacific, by intense widespread mid-plate volcanism. The demise of reefs on numerous Pacific atolls has left a large number of guyots. This synthesis explores the possibility that the middle Cretaceous demise of mid-Pacific and other carbonate-bank communities, and related extinctions elsewhere, reflects sporadic, possibly volcanogenic, upwelling of anoxic water. The upwelling would have had to be rapid enough to overwhelm surface reoxygenation, and frequent/widespread enough to suppress recolonization. According to the Hallock-Schlager hypothesis, upwelling of nutrient-rich waters destroyed the carbonate-bank communities; nutrient excess and anoxia may well have worked together. The globally widespread presence of at least mid-depth anoxia (±H 2 S?) is reasonably well established (the Cenomanian/Turonian Oceanic Anoxic Event perhaps was an extreme example), as is the mortality of modern benthic invertebrates at O 2 levels below about 0.15 ml/l. Recolonization times for modern reef corals imply that upwelling catastrophes, if localized, need have recurred on approximately decade time scales. Previously postulated Cretaceous sea-level fluctuations, raising sea level at rates on the order of 5-10 m in 10 4 -10 5 yr, would then have sufficed to carry the intermittently stressed reef surfaces below the ∼5- to 10-m-deep high-carbonate-production zone, below which reef recovery chances diminish. Thus, the episode of frequent (localized but widespread) upwelling events need have lasted only about 10 4 -10 5 yr. Although intermittent deep-water ventilation is known to have occurred on Milankovitch time scales during the Cretaceous, thereby bringing anoxic and/or nutrient-rich waters to the surface, geologic (and ocean modeling) evidence is necessary to establish whether a significant amount of rapid upwelling did (or could) occur. Because mid-plate submarine volcanism was widespread and intense in the middle Cretaceous Pacific, volcanogenic/hydrothermal heating and entrainment of nutrient-rich anoxic waters is one possible mechanism for catastrophic buoyant upwelling, rudistid bank extinction, and guyot formation. The efficacy of this process depends principally on the buoyancy (that is, heat energy) flux and the buoyancy (Brunt-Vaisala) frequency. Typical hydrothermal plumes along the Mid-Oceanic Ridge axis rise only to 150- to 400-m heights above the sea floor, but megaplumes rising 1,000 m are known. Simple plume models imply that only basalt eruptions comparable to the largest historically known would be sufficiently energetic (10 12 -10 14 J/s) to bring bottom water to the ocean surface from normal Mid-Oceanic Ridge axial depths (3 km) over much of the present ocean; however, these models are conservative through neglect of momentum overshoot and gas exsolution. To the extent that higher eruption rates and lower Brunt-Vaisala frequencies (due to lower vertical temperature gradients) prevailed in the middle Cretaceous Pacific, anoxic deep waters would more readily have reached the ocean surface. Most Cretaceous guyots are now in the North Pacific but were carried across the equator by Pacific plate motion; normal tendencies to upwelling and oxygen deficiency along the equator would have made the carbonate banks most vulnerable during their equatorial transit.

Experimental investigation of transient pore pressure behaviour in soils due to gas exsolution : Sobkowicz, J C; Morgenstern, N R Proc International Symposium on Prediction and Performance in Geotechnical Engineering, Calgary, 17–19 June 1987P267–275. Publ Rotterdam: A A Balkema, 1987

Experimental investigation of transient pore pressure behaviour in soils due to gas exsolution : Sobkowicz, J C; Morgenstern, N R Proc International Symposium on Prediction and Performance in Geotechnical Engineering, Calgary, 17–19 June 1987P267–275. Publ Rotterdam: A A Balkema, 1987

Highwall geotechnical engineering at Syncrude

Abstract Local adverse structural geology combined with gas exsolution from bitumen, high pore pressures and stress relief contribute to an extensive highwall geotechnical program at Syncrude. Site investigations along the ten kilometres of highwall are required to define any adverse geotechnical conditions that may lead to any of the five recorded modes of failures. A geotechnical organization comprising of highwall monitoring engineers, instrumentation engineers, highwall analysis and design engineers and geotechnical project engineers is required to ensure the risk of highwall instability on dragline mining is kept to a minimum. Alternate highwall designs, remedial stabilization plans plus instrumentation and highwall monitoring programs are discussed along with the impact of highwall instability on the dragline mining operation.

Lava fountain heights at Pu'u 'O'o, Kilauea, Hawaii: Indicators of amount and variations of exsolved magma volatiles

We review the factors determining lava fountain heights in Hawaiian‐style basaltic eruptions such as those occurring during the recent series of eruptive episodes at the Pu'u ‘O’ o vent, east rift zone of Kilauea Volcano, Hawaii. Numerical solutions to the equations describing the fluid dynamics of such eruptions predict that lava fountain heights, which are indicators of the velocity of magmas in vents, should be controlled much more strongly by amounts of exsolved volatiles than by any other factors. The next most important factor is the width of the conduit system, which determines frictional losses and can be characterized by the volume flux of magma. The diameter of the surface vent required to accommodate a given discharge (i.e., instantaneous volume rate of eruption) is also a function of exsolved magma volatile content but is less dependent on this factor than is the fountain height. We simulate fountain heights for typical discharges at Pu'u ‘O’ o and find implied exsolved gas contents very close to those determined by other methods. The corresponding predicted vent diameters are comparable with (and somewhat smaller than) the observed diameter of the Pu'u ‘O’ o vent. The discrepancy is probably a consequence of processes occurring in the lava pond which is present over the vent during eruptions. The variation of fountain height with time during eruptive episodes can be used as a measure of changing exsolved magma volatile content, which can be related to processes occurring in the subsurface dike system.

Residual gravity changes and eruption magnitudes

Research during the last four decades has demonstrated that systematic temporal gravity changes are associated with eruptive activity. These gravity changes are caused by the free-air effect of elevation changes induced by magma bodies and by mass redistributions. Where gravity change data are accompanied by concurrent elevation control, observed gravity changes can be corrected for the free-air effect giving residual gravity changes. Residual gravity changes reflect only mass redistributions. At most volcanoes residual gravity changes can be accounted for by the addition of mass during pre-eruption time, and by the removal or addition of mass during post-eruption time. At explosive volcanoes, however, the data show mass must be removed from near surface magma systems during preeruptive time. The only reasonable mechanism acounting for these negative pre-eruption mass redistributions is displacing dense country rock or magma with gas bubbles in vesiculated magma. Application of Gauss' theorem to areal residual gravity changes and an assumed density for the displaced material, give an estimate of the volume of exsolved gas contained within the magma body. The kinetic energy released in explosive magmatic eruptions, to a first approximation, is caused by the explosive expansion of exsolved and dissolved gases contained within magma. If the density and temperature of magma, and if the amount, pressure, and composition of volatiles contained in magma bodies are known (or assumed), then the kinetic energy released by explosive gas decompression can be computed (or estimated ) using simple thermodynamic relationships. Kinetic energy estimates for explosive magmatic eruptions computed from muzzle velocities, and computed for isothermal and adiabatic gas decompression using assumed volatile contents, pressures, and temperatures give similar results. Minimum pre-eruption exsolved volatile contents estimated from negative pre-eruption mass deficits and assumed magma densities, temperatures, and pressures were used to compute predicted kinetic energies for five eruption-gravity change sequences at Pacaya, Mihara, Usu, and Mt. St. Helens volcanoes. Although the data are too few to be conclusive, they do indicate the possibility of forecasting eruption magnitudes at some volcanoes using precursory gravity changes.

Pipe vesicles—An alternate model for their origin

Most pipe vesicles occur near the base of basaltic flows where they are interpreted to be traces left by ascending gas bubbles. Some basaltic pillows also contain pipe vesicles, but in these pillows pipe vesicles have a radial distribution, indicating that buoyancy cannot be a factor in their formation. Pipe vesicles do not extend through chilled glassy margins at the base of flows or rims of pillows, but rather, they form only where there has been significant crystallization, indicating that the gas forming these vesicles is exsolved from the lava and is not derived from an external source. Pipe vesicles in flows and pillows are proposed to form by the exsolution of gas onto bubbles that are attached to the zone of solidification. As this zone advances into the cooling lava, continued exsolution of gas causes the bubbles to grow normal to the solidification front as pipes. Gas tubes formed in a similar manner can be seen in most ice cubes. Lava near the advancing tip of a pipe vesicle cools more rapidly than elsewhere because heat is able to transfer along the pipe by radiation. Pipe vesicles, therefore, grow into the lava as “cold fingers.” They cease growing when separate gas bubbles nucleate ahead of them in the ever broadening zone of solidification. Lava need contain no more than 0.01 wt% H 2 O for pipe vesicles to form in this way.

Exsolution of H2O, CO2, and S during eruptive episodes at Kilauea Volcano, Hawaii

A model for the exsolution of H2O, CO2, and S from Kilauea magma as it rises from a shallow crustal reservoir predicts that vigorous exsolution occurs only after magma has ascended to shallow depths (≲ 150 m lithostatic) where pressures are <3 MPa (30 bars). Exsolved vapor at saturation (30–100 MPa) is CO2‐rich (70 vol %, maximum), but H2O increases and ultimately predominates in the vapor as pressure drops. The model indicates that most H2O remains dissolved, however, down to pressures as low as 2–3 MPa (20–30 bars) rather than being exsolved abundantly below 8 MPa (80 bars) as is commonly assumed. Most CO2 is exsolved above 10 MPa, whereas most S, like H2O, is exsolved below 2–3 MPa (<100–150 m lithostatic). The critical condition for major outgassing and disruption of magma into spray is reached at 1.0–0.6 MPa (∼40–25 m lithostatic). Gas exsolution apparently continues after disruption. The total quantity of exsolved volatiles as predicted by the exsolution model agrees with that predicted by models of fire fountain dynamics. At 0.3–0.4 MPa (≲20 m lithostatic), the exsolved gases resemble volcanic gases of east rift zone eruptions and coexist with melts containing volatiles at concentrations similar to those observed in fountain spatter. Exsolution from coalesced spatter of previously disrupted melt produces residual gases that are H2O‐rich and sharply depleted in CO2. The low pressures and shallow depths required for extensive exsolution imply that magma must be transported nearly to the surface before disruption, outgassing, and fountaining are possible. Therefore eruption forecasting techniques based on detection of volatiles outgassed from ascending magma may not provide useful advance warning at Kilauea and other tholeiite basalt systems.

Eruptive style and construction of shallow marine mafic tuff cones in the Narakay Volcanic Complex (Proterozoic, Hornby Bay Group, Northwest Territories, Canada)

Abstract The Early Proterozoic (1663 Ma) Narakay Volcanic Complex, exposed in Great Bear Lake (Northwest Territories, Canada), is a bimodal suite of basalt and rhyolite erupted in a continental setting and consisting largely of pyroclastic rocks interlayered with shallow marine sedimentary rocks of the Hornby Bay Group. Mafic pyroclastic rocks consist of lapilli tuff, tuff, tuff breccia and agglomerate that represent the remnants of small subaerial tuff cones (0.5 to 2 km in diameter) that in most cases have subsided into the volcanic conduit. Stratification styles, sedimentary structures and grain morphologies in pyroclastic rocks reflect variations in the water:magma ratio during eruptions and have been used to help elucidate eruptive mechanisms and reconstruct volcanic edifices. Basaltic pyroclasts are commonly bounded by fracture surfaces and are morphologically similar to modern pyroclasts produced by thermal quench fragmentation or steam-blast disruption of magma. Most fragments have low vesicularity and scoria is only locally abundant which indicates that eruptive energy was supplied mostly by water—melt interaction rather than exsolution of magmatic gases. Cored bombs and lapilli, fusiform bombs, and pyroclasts similar in texture to those of Strombolian cinder and agglutinate spatter, are uncommon but are stratigraphically widespread and imply the occurrence of Strombolian eruptions, presumably when water access to the vent was impeded. Massive bedding is typical of the tuffs and, in addition to the poorly sorted ash-rich nature of the tuffs, implies deposition from water- and/or steam-rich hydrovolcanic eruption clouds and cypressoid jets by airfall and dense pyroclastic flows. Uncommon well-stratified and sorted ash and lapilli tuff record airfall and pyroclastic flow(?) deposition from eruption clouds rich in magmatic gases. Base surge deposits are uncommon and occur only in the subaerial portion of a sequence of tuffs inferred to record the progradation of a cone-margin surge platform into standing water. Few of the tuff cone deposits display a systematic vertical sequence of stratification styles, structures and grain morphologies. This indicates that either the eruptive style varied irregularly between hydrovolcanic and Strombolian and/or that pyroclasts of different origin were mixed during eruptions.

Hydrothermal eruptions in the Waiotapu geothermal system, New Zealand; their origin, associated breccias, and relation to precious metal mineralization

The distribution, petrography, and mineralogy are described for hydrothermal eruption breccias in the active Waiotapu geothermal system, New Zealand. These surficial deposits are characterized by variable clast abundance, size, and angularity; a poorly sorted, matrix-supported texture; an absence of primary volcanic material; and a variable degree of hydrothermal alteration and brecciation between clasts and deposits. Clasts in some deposits provide evidence for transport from about a 300-m depth and eruption velocities of 200 to 300 m/s. Previously published discussions of the origin of these types of eruption, or equivalent breccias in epithermal mineralized districts, have focused on the development of substantial fluid overpressures in the upper silicified portion of a hydrothermal system. In this paper an alternative mechanism is proposed which requires only local sealing of near-surface discharge channels and the transmission of deeper reservoir pressures to the sealed area by the evolution of a compressible cap of exsolved gas (dominantly CO 2 ). Many of the Waiotapu eruption vents have been flooded by ground water, but a number, including the Champagne Pool which was formed 900 years ago, continue to discharge chloride water directly from the deeper hydrothermal reservoir. Precipitates formed within this pool are enriched in arsenic, antimony, thallium, and mercury and are ore grade with respect to gold and silver. Two chemical environments are recognized in and below such vents which (subsequent to the eruption) may lead to precious metal deposition: (1) boiling and H 2 S loss in the fracture network and conduit below the eruption crater and (2) coupled surface heat loss and pH buffering in the crater pool which leads to arsenic sulfide precipitation with coprecipitation of gold and other metals. It is argued that about one hundred thousand ounces of gold have deposited beneath Champagne Pool since its formation but that the overall transport and depositional efficiency of the Waiotapu system is only 10 percent. In 10,000 years this is sufficient for the deposition of 1- to 6-million-ounce orebodies of the type under exploitation at Round Mountain (Nevada), McLaughlin (California), and in other epithermal districts. Exploration programs for epithermal precious metal deposits should be based on our knowledge of the structural and chemical characteristics of active geothermal systems. Coupling patterns of trace element distribution, alteration assemblages, and fluid inclusion data with the location of hydrothermal breccias (vents and/or surficial deposits) provides a strategy for the targeting of exploration drilling by identifying flow paths along which boiling has occurred.

Magma mixing in a main stage of formation of Montagne Pelée: the Saint Vincent-type scoria flow sequence (Martinique, F.W.I.)

Contrasting with the first stage of activity at Montagne Pelée, which built an andesitic volcano chiefly formed of brecciated lava flows and pyroclastic breccias, the second main stage (> 40,000 to 20,000 years B.P.) is characterized by the prevalence of scoria flows of more mafic composition. Many cross-sections of the deposits of stage 2 show an upward magmatic evolution from early acid andesites (plinian air-fall and pumice flow deposits) to late basaltic andesites (scoria flow deposits). Heterogeneous “banded” rocks, complex mineralogical associations and mineralogical disequilibrium features in the whole series are explained by mechanical mixing of contrasting magmas. Reversed zoning characterized plagioclase phenocrysts of both the acid and basic terms and also pyroxene phenocrysts of the mafic terms. Grossly uniform composition of residual glass in most macroscopically homogeneous rocks implies that chemical exchanges between liquids and local hybridization occurred. Plutonic rocks with cumulate textures are found in upper units of the sequence. Their phenocrysts (anorthite, salitic clinopyroxene, pargasitic hornblende and Ti-magnetite) are in equilibrium with a subalkaline basaltic interstitial liquid. The same phases are commonly present as xenocrysts in the basaltic andesites. It is inferred that basaltic andesites of stage 2 were firstly derived from a subalkaline basalt by crystal fractionation. Mixing of the basaltic andesites (52% SiO2) with acid andesites (62% SiO2) occurred in late stage of evolution of the series, producing heterogeneous rocks of intermediate composition. Successive eruption of acid up to mafic magmas, dominant amount of mafic rocks in the sequence and short duration of the magma mixing event shown by narrow rims of the reversely zoned phenocrysts, suggest that mixing was produced by intrusion of mafic magma into a differentiated magma body. We propose that hydrostatic overpressure induced by a high rate of intrusion triggered the eruption. Thus, magma mixing would be due to a large volume of mafic magma passing through the resident magma. Rapid admixture of the two batches increased gas exsolution, resulting in a highly explosive eruption. Magma mixing can thus explain the explosive dynamics of the basaltic andesites eruptions, which surprisingly contrast with the relatively effusive style of the preceding andesitic stage.

Vanadium sublimates from the fumaroles of Izalco volcano, El Salvador

Prior to the recent discovery of high-temperature vanadium sublimates at the fumaroles of Izalco volcano, El Salvador, the only vanadium minerals known were those formed at low-temperature surface conditions. Six new vanadium compounds have already been discovered at Izalco volcano, and five of these have been elevated to mineral status. Two of the minerals are among the most vanadium-rich compounds known in nature. Field studies and studies of the crystal chemistry of the sublimate phases have allowed delineation of the conditions of exsolution of vanadium-bearing gases from the magma and conditions of sublimation of the vanadium compounds. A vanadium halogen or oxyhalogen gas phase was exsolved from a basaltic magma when the magma was 550 m below the summit crater. As the gas rose to the fumaroles and mixed with the atmosphere, the oxygen fugacity rose from that of the magma to at least 10 −3,5 bar. At the fumaroles, the vanadium compounds sublimated at various temperatures, but a minimum temperature of 760° can be determined for one of the phases based on geothermometry determined from phase relations in the system CuO-V 2O 5. The studies have demonstrated that a vanadium-bearing gas can segregate from a magma with normal basaltic concentrations of vanadium.

An experimental investigation of volatile exsolution in evolving magma chambers

Previous laboratory experiments investigating the fluid dynamics of replenished magma chambers have been extended to model effects resulting from the release of gas. Turbulent transfer of heat between a layer of dense, hot and volatile-rich mafic magma overlying cooler more evolved magma can lead to crystallization and exsolution of volatiles in the lower layer. Small gas bubbles can cause the bulk density to decrease to that of the upper layer and thus produce sudden overturning and initiate mixing, followed by further exsolution of gas and explosive eruption. These processes have been modelled in the laboratory using a chemical reaction between sodium or potassium carbonate and nitric acid to release small bubbles of CO 2 . We have investigated both the initial overturning produced by gas release in the lower layer, and the subsequent evolution of gas due to intimate mixing of the two layers. The latter experiments, in which the reactants remained isolated in the two layers until overturning occurred, demonstrated unambiguously that the fluxes of chemical components across the interfaces between convecting layers are very slow compared to the flux of heat. This shows that the evolution of layers of magma of different origins and composition can take place nearly independently of each other. The magmas can coexist in the same stratified chamber, until their bulk densities become equal and they mix together. The processes illustrated in these experiments could occur in H 2 O-bearing magmas such as in the calcalkaline association and in CO 2 -bearing mafic magmas such as in silica undersaturated suites.

Hot isostatic pressing to remove porosity & creep damage

The creep life of components operating at high temperature is frequently limited by the accumulation of cavities and voids on grain boundaries. In addition, the structural integrity of castings is degraded by micropores resulting from shrinkage or gas exsolution during solidification. All such ‘defects’ can be healed by heat treatments under hydrostatic pressure of inert gas (Hot Isostatic Pressing, HIP). Here we consider the theoretical contribution of hydrostatic pressure to the sintering of cavities and pores, and show how parametric methods may be used to predict the required conditions. Experimental results are presented for the healing of cavities and pores in a dilute nickel alloy and a nickel-base superalloy, and it is shown how such procedures may be used to improve or regenerate creep properties.

Ascent and eruption of basaltic magma on the Earth and Moon

Geological and physical observations and constraints are applied to the development of a model of the ascent and emplacement of basaltic magma on the earth and moon. Mathematical models of the nature and motion of gas/liquid mixtures are developed and show that gas exsolution from terrestrial and lunar magmas commonly only occurs at shallow depths (less than 2 km); thus the ascent of bubble‐free magma at depth can be treated separately from the complex motions caused by gas exsolution near the surface. Magma ascent is related to dike or conduit width; a lower limit to width is determined by the presence of a finite magma yield strength or by excessive magma cooling effects related to magma viscosity. For terrestrial basalts with negligible yield strengths and viscosities greater than 102 Pa s, widths in the range 0.2–0.6 m are needed to allow eruptions from between depths of 0.5–20 km. Fissure widths of about 4 m would be needed to account for the output rates estimated for the Columbia River flood basalt eruptions. As the magma nears the surface, bubble coalescence will tend to occur, leading to intermittent explosive strombolian‐style activity. For commonly occurring lunar and terrestrial basalts the magma rise speed must be greater than 0.5–1 m/s if strombolian activity is to be avoided and relatively steady fire fountaining is to take place. Terrestrial fire fountain heights are dictated by the vertical velocity of the magma/gas dispersion emerging through the vent, increasing with increasing magma gas content and mass eruption rate, and decreasing with increasing magma viscosity. Terrestrial fire fountain heights up to 500 m imply the release of up to 0.4 wt % water from the magma, corresponding to initial water contents up to 0.6 wt %. The presence of extremely long lava flows and sinuous rules on the moon has often been cited as evidence for very high extrusion rates and thus a basic difference between terrestrial and lunar magmas and crustal environments. However, the differences between terrestrial and lunar magma rheologies and crustal environments do not lead to gross differences between the effusion rates expected on the two planetary bodies, for similar‐sized conduits or fissures. Thus the presence of these features implies only that tectonic and other forces associated with the onset of some lunar eruptions were such as to allow wide fissures or conduits to form. The surface widths of elongate fissure vents need be no wider than 10 m to allow mass eruption rates up to 10 times larger than those proposed for terrestrial flood basalt eruptions; 25‐m widths would allow rates 100 times larger. It therefore appears unlikely that source vents on the moon with widths greater than a few tens of meters represent the true size of the unmodified vent. The main volatile released from lunar magmas was probably carbon monoxide, released in amounts proportionally less than terrestrial magmas by more than an order of magnitude. However, decompression to the near‐zero ambient lunar atmospheric pressure causes much greater energy release per unit mass and this, coupled with vertical and horizontal expansion of the gas, suggests a much more efficient use of the available gas on the moon than on the earth. Some amount of magma disruption must always have taken place in lunar eruptions unless the gas content was truly zero or the magma possessed an appreciable yield strength. Pyroclastic deposits, such as the extensive lunar dark mantling material, could be produced from a single source vent and extend to diameters of up to 200 km. Such deposits could result either from steady eruptions at high effusion rates (with less than 1% of the magma disrupted into submillimeter droplets) or low effusion rate eruptions in which strombolian activity occurred. The wide dispersal of pyroclastic debris is a result of small particles being locked into the expanding gas cloud. Finally, we consider the details of basaltic eruption processes on the moon and predict the nature and geometry of several types of volcanic landforms (flows, pyroclastic blankets and cones, cinder and spatter ridges, etc.) that should result from specific eruption conditions.

Heat-Flow Measurements in 17 Perialpine Lakes

Thirty-eight heat-flow measurements were carried out in 17 perialpine lakes to supplement the scanty information concerning the thermal conditions in the crust underlying central Europe. A probe of the corer type was built which allowed the measurement of 11 temperatures at equally spaced subbottom intervals to a maximum penetration of 10.5 m. Measurements of this type were made in Lake Constance, Greifensee, Lake Zurich, Lake Zug, Lake Lucerne, Baldeggersee, Lake Biel, Lake Murten, Lake Neuchâtel, Lake Geneva, Lac d9Annecy, Lac de Bourget, Lago Maggiore, Lago di Lugano, Lago di Como, Lago d9Iseo, and Lago di Garda. After retrieval of the probe, a sediment core was collected for samples on which thermal conductivity was measured with the needle probe method after treating the material to remove exsolved gas pubbles. Continuous seismic-reflection and reflection profiles in all of these lakes provided a set of parameters for gradient correction due to heat-flow refraction. Additional corrections were applied on the basis of palynologically determined sedimentation rates and for the influence of the last period of glaciation. Final heat-flow values obtained in this manner are given. The lesser values are generally found in the more western lakes, north of the Alps; the higher values are found in the more eastern lakes, north and south of the Alps. The higher values are partially confirmed by earlier measurements in lakes but show considerable discrepancy with measurements from the central Alps. This could be explained by the underthrusting of a cold wedge under the Aar and Gotthard Massif in the late Miocene and early Pliocene, which then would lower the temperature gradient in the central Alps.

Diagenesis and Methane Generation in Upper Cretaceous Gammon Shale, Northern Great Plains, United States: ABSTRACT

In the northern Great Plains, isotopically light methane is entrapped at shallow depths in marine rocks of Late Cretaceous age. Products of early diagenetic decomposition of organic matter in the Gammon Shale support the view that the gas is biogenic and formed at shallow depths early in the burial history of the sediments. This interpretation implies widespread gas occurrence and is consistent with a larger gas resource figure than alternative interpretations suggest. The Gammon Shale was deposited offshore during a major regression of the Late Cretaceous epeiric sea. The sediment-water interface was oxygenated, and soft-bodied organisms burrowed the silt-clay sediment. Organic matter was sufficiently abundant for oxygen depletion at shallow depths. Bacterial sulfate reduction occurred quickly and resulted in the formation of framboids and octahedra of pyrite. Abundant concretions and discrete crystals of siderite began forming within tens of meters of the sediment surface. Interstitial waters became saturated with methane, and a free gas phase, held in siltstone layers by capillary forces, inhibited silicate diagenesis. Methane generation probably continued to burial depths of hundreds of meters. At the maximum burial depth (1,200 to 1,500 m), int rstitial waters contained their maximum dissolved methane, and silt layers still contained free gas. Cenozoic uplift and erosion permitted gas exsolution. Exsolved gas combined with free methane already in the reservoirs to form the gas being currently explored and extracted. End_of_Article - Last_Page 929------------

Explosive volcanic eruptions -- IV. The control of magma properties and conduit geometry on eruption column behaviour

Summary Plinian air-fall deposits and ignimbrites are the principal products of explosive eruptions of high viscosity magma. In this paper, the flow of gas/pyroclast dispersions and high viscosity magma through various magma chamber/conduit/vent geometries is considered. It is argued that after the first few minutes of an eruption magma fragmentation occurs at a shallow depth within the conduit system. Gas pressures at the fragmentation level are related to exsolved gas contents by consideration of the exsolution mechanism. The sizes of blocks found near vents imply that gas velocities of 200 to 600 m s−1 commonly occur. These velocities are greater than the effective speed of sound in an erupting mixture (90-200 m s−1) and the transition from subsonic to supersonic flow is identified as occurring at the depth at which the conduit has its minimum diameter. The range of values of this minimum diameter (∼ 5 to ∼ 100 m) is estimated from observed and theoretically deduced mass-eruption rates. The energy and continuity equations are solved, taking account of friction effects, for numerous geometries during the evolution, by wall erosion, of a conduit. Conduit erosion ceases, near the surface, when an exit pressure of one atmosphere is reached. Eruption velocities are found to depend strongly on exsolved magma gas content and weakly on radius of conduit and friction effects. Assuming water as the main volatile phase, velocities of 400-600 m s−1 for plinian events imply magma water contents of 3-6 per cent by weight. Three scenarios are presented of eruptions in which: (1) conduit radius increases but gas content remains constant; (2) conduit radius increases and gas content decreases with time; and (3) conduit radius remains fixed and gas content decreases. These models demonstrate that the reverse grading commonly observed in plinian air-fall deposits is primarily a consequence of conduit erosion, which always results in increasing eruption intensity and eruption column height with time. The models also show that a decrease in gas content as deeper levels in a magma chamber are tapped or an increasing vent radius as conduit walls are eroded leads to the prediction of a progression from air-fall activity through ignimbrite formation to cessation of eruption and caldera collapse.

Thermal history of primordial metal grains

Certain types of meteorites contain grains generally considered to have been formed by condensation from the space medium in the early solar system. The highest temperatures at which such grains can have existed for any given length of time can be determined from diffusion-controlled phenomena such as exsolution of insoluble gases and solids and interdiffusion of reactive components. Among systems of the latter kind are platinum spherules occluded in nickel-iron grains which in turn occur in the interior of refractory silicate crystals. The lack of observable interdiffusion of platinum and nickel-iron, together with the known interdiffusion of these metals show that the nickel-iron and the refractory silicate could not have been at a temperature of 1600 K for more than a fraction of a second. For growth periods of a day and a year the highest temperatures allowed are 900 K and 780 K, respectively. Lower bounds are set by the silicate glass-crystal transition temperature.

On the removal of pores from castings by sintering

The causes, sizes, and distribution of porosity in castings have been reviewed and quantitatively evaluated for several important modes of alloy solidification. In general, gas exsolution is found to be the most probable cause of porosity in castings which solidify in either a cellular or dendritic fashion. On the other hand, solidification alone may cause porosity creation if the interdendritic liquid metal cannot feed the solidification shrinkage. This effect may be enhanced by gas exsolution.

Crystallization of tholeiitic basalt in Alae Lava Lake, Hawaii

The eruption of Kilauea Volcano August 21–23, 1963, left 600,000 cubic meters of basaltic lava in a lava lake as much as 15 meters deep in Alae pit crater. Field studies of the lake began August 27 and include repeated core drilling, measurements of temperature in the crust and melt, and precise level surveys of the lake surface. The last interstitial melt in the lake solidified late in September 1964; by mid August 1965 the maximum temperature was 690°C at a depth of 11.5 meters. Pumice air-quenched from about 1140°C contains only 5 percent crystals — clinopyroxene, cuhedral olivine (Fo 80), and a trace of plagioclase, (An 70). Drill cores taken from the zone of crystallization in the lake show that olivine continued crystallizing to about 1070°C; below that it reacts with the melt, becoming corroded and mantled by pyroxene and plagioclase. Below 1070°C, pyroxene and plagioclase crystallized at a constant ratio. Ilmenite first appeared at about 1070°C and was joined by magnetite at about 1050°C; both increased rapidly in abundance to 1000°C. Apatite first appeared as minute needles in interstitial glass at 1000°C. Both the abundance and index of refraction of glass quenched from melt decreased nearly linearly with falling temperature. At 1070°C the quenched lava contains about 65 percent dark-brown glass with an index of 1.61; at 980°C it contains about 8 percent colorless glass with an index of 1.49. Below 980°C, the percentage of glass remained constant. Progressive crystallization forced exsolution of gases from the melt fraction; these formed vesicles and angular pores, causing expansion of the crystallizing lava and lifting the surface of the central part of the lake an average of 19.5 cm. The solidified basalt underwent pneumatolitic alteration, including deposition of cristobalite at 800°C, reddish alteration of olivine at 700°C, tarnishing of ilmenite at 550°C, deposition of anhydrite at 250°C, and deposition of native sulfur at 100°C. Ferric-ferrous ratios suggest that oxidation with maximum intensity between 550°C and 610°C moved downward in the crust as it cooled; this was followed by reduction at a temperature of about 100°C. The crystallized basalt is a homogeneous fine-grained rock containing on the average 48.3 percent by volume intergranular pyroxene (augite > pigeonite), 34.2 percent plagioclase laths (An60 70), 7.9 percent interstitial glass, 6.9 percent opaques (ilmenite > magnetite), 2.7 percent olivine (Fo70 80), and a trace of apatite. Chemical analyses of 18 samples, ranging from initially quenched pumice to lava cored more than a year after the eruption from the center and from near the base of the lake, show little variation from silica-saturated tholeiitic basalt containing 50.4 percent SiO2, 2.4 percent Na2O, and 0.54 percent K2O. Apparently there was no significant crystal settling and no appreciable vapor-phase transport of these components during the year of crystallization. However, seven samples of interstitial liquid that had been filter-pressed into gash fractures and drill holes from partly crystalline mush near the base of the crust show large differences from the bulk composition of the solidified crust—lower MgO, CaO, and Al2O3; and higher total iron, TiO2, Na2O, K2O, P2O5, and F, and, in most samples, SiO2. The minor elements Ba, Ga, Li, Y, and Yb and possibly Cu tend to be enriched in the filter-pressed liquids, and Cr and possibly Ni tend to be depleted.