Circulation of bubbly magma and gas segregation within tunnels of the potential Yucca Mountain repository

Following an intersection of rising magma with drifts of the potential Yucca Mountain nuclear waste repository, a pathway is likely to be established to the surface with magma flowing for days to weeks and affecting the performance of engineered structures located along or near the flow path. In particular, convective circulation could occur within magma-filled drifts due to the exsolution and segregation of magmatic gas. We investigate gas segregation in a magma-filled drift intersected by a vertical dyke by means of analogue experiments, focusing on the conditions of sustained magma flow. Degassing is simulated by electrolysis, producing micrometric bubbles in viscous mixtures of water and golden syrup, or by aerating golden syrup, producing polydisperse bubbly mixtures with 40% of gas by volume. The presence of exsolved bubbles induces a buoyancy-driven exchange flow between the dyke and the drift that leads to gas segregation. Bubbles segregate from the magma by rising and accumulating as a foam at the top of the drift, coupled with the accumulation of denser degassed magma at the base of the drift. Steady-state influx of bubbly magma from the dyke into the drift is balanced by outward flux of lighter foam and denser degassed magma. The length and time scales of this gas segregation are controlled by the rise of bubbles in the horizontal drift. Steady-state gas segregation would be accomplished within hours to hundreds of years depending on the viscosity of the degassed magma and the average size of exsolved gas bubbles, and the resulting foam would only be a few cm thick. The exchange flux of bubbly magma between the dyke and the drift that is induced by gas segregation ranges from 1 m³ s⁻¹, for the less viscous magmas, to 10⁻⁸ m³ s⁻¹, for the most viscous degassed magmas, with associated velocities ranging from 10⁻¹ to 10⁻⁹ m s⁻¹ for the same viscosity range. This model of gas segregation also predicts that the relative proportion of erupted degassed magma, that could potentially carry and entrain nuclear waste material towards the surface, would depend on the value of the dyke magma supply rate relative to the value of the gas segregation flux, with violent eruption of gassy as well as degassed magmas at relatively high magma supply rates, and eruption of mainly degassed magma by milder episodic Strombolian explosions at relatively lower supply rates.     

Injection and arrest of dykes: implications for volcanic hazards

Dykes are the principal channels through which magma reaches the surface in volcanic eruptions. For this reason dykes observed in the field are commonly assumed to be feeders to lava flows. The actual proportion of dykes reaching the surface is, however, poorly known. In order to develop models for the purpose of estimating volcanic hazard, this proportion must be known. This follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes, but also the probability of the injected dykes being arrested. This paper presents field data on several thousand dykes from Iceland and Tenerife (Canary Islands) indicating that many, and probably most, dykes become arrested at various crustal levels and never reach the surface to feed eruptions. Using the results of analytical and numerical models, it is shown that, for common loading conditions, the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest. This means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested. The implication is that during periods of unrest in volcanoes, the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber.     

Determining magma flow in sills,dykes and laccoliths and their implications for sill emplacement mechanisms

Three-dimensional seismic data from the Faeroe-Shetland Basin provides detailed information on the relationships between sills, dykes, laccoliths and contemporaneous volcanic activity. The data shows that sills are predominantly concave upwards, being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet. Such morphologies are only partially modified by pre-existing faults. Sills can be sourced from dykes or the steep climbing portions of deeper sills. Both sills and dykes can provide magma to overlying volcanic fissures and sills can be shown to feed shallow laccoliths. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. As an individual sill can consist of several leaves emplaced at different stratigraphic levels, and as a sill or dyke can provide magma to volcanic fissures, other sills and laccoliths, the data suggests that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement. Instead, the data suggests that the presence of lithological contrasts, particularly ductile horizons such as overpressured shales may permit sill formation at any level below the neutrally buoyant level. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Ken Thomson–deceased, April 2007     

Middle Miocene to Quaternary primary basalt and high magnesian andesite magmas of North Hokkaido, Japan: Source mantle characteristics and degrees of partial melting

Abstract   Middle Miocene to Quaternary primitive basalts and high magnesian andesite (HMA) in North Hokkaido resulted from three periods of intense volcanism; early-stage (12–10 Ma), middle-stage (9–7 Ma) and late-stage (3–0 Ma). Based on the chemical compositions of olivines and chromian spinels and bulk chemistry of the primitive rocks, we examined depths of segregation of the calculated primary magmas and the degrees of partial melting of the source mantle. In the context of asthenospheric mantle upwelling, petrological data from the present study can be accounted for by the secular change in the depth of magma segregation from the upwelled asthenospheric mantle, which is composed of fertile peridotite. Thus, the early-stage primary magmas were generated by higher degrees of partial melting of the shallower part of hot asthenospheric mantle, whereas the middle- and late-stage primary magmas resulted from lower degrees of partial melting of a deeper part of the asthenospheric mantle. The early-stage HMA magma was generated by partial melting of the remnant subcontinental lithospheric mantle composed of refractory peridotite. This melting might have resulted from an increased geothermal gradient caused by upwelling of hot asthenosphere.     

Modelling the dynamics and thermodynamics of volcanic degassing

 The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO₂–saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission rate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power, and inversely proportional to the degassed magma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this dampens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the ∼700 kg s^–1 magmatic water flux can be modelled with a 4–10 m radius conduit, degassing 20–100% of the available water and all of the 1 to 4 vol.% CO₂ chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the ∼500 kg s^–1 magmatic water flux can be modelled with a 22–60 m radius conduit, degassing ∼2–90% of the available water and all of the 0.1 to 3 vol.% CO₂ chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes. Received: 26 September 1997 / Accepted: 11 July 1998     

New unspiked K–Ar ages of volcanic rocks of the central and western sector of the Aeolian Islands: reconstruction of the volcanic stages

A geochronological study of the Filicudi, Salina, Lipari and Vulcano Islands (Aeolian Archipelago) using the unspiked potassium–argon technique provides new age data which, combined with stratigraphic correlation, better constrain the temporal evolution of volcanism. The unspiked K–Ar age of the oldest exposed lavas on Filicudi, 219±5 ka, is significantly younger than the previous estimation of 1.02 Ma. In the general context of Aeolian volcanism, this new date suggests that the volcanism of the western sector of the Aeolian Archipelago is younger than previously thought. Geochronological data point out on the rapid transition from calc–alkaline to potassic volcanism. The distribution of the K–Ar ages within the Salina–Lipari–Vulcano group shows that the volcanism started on Lipari and propagated over time northward on Salina and southward on Vulcano. Geochronological and geophysical data suggest that the onset of volcanism in the central sector of the Aeolian Arc may be due to a mantle upwelling structure located below Lipari. A change in the style of the eruptions occurred in the Salina–Lipari–Vulcano system at about 100 ka from the present. Low-energy magmatic eruptions occurred between 188 and about 100 ka. From about 100 ka to the present, higher-energy eruptions and low-energy events due to magma–water interaction also occurred. This change in the style of activity, together with the appearance of evolved products (i.e. rhyolites) during the last 50 ka, is consistent with the formation of magmatic reservoirs located at shallower depth with respect to those of the 188–100-ka period. The new geochronological data and available petrological models reveal that a change in the deep source of the primary magmas occurred in a relatively short time interval.     

Emplacement and arrest of sheets and dykes in central volcanoes

Sheet intrusions are of two main types: local inclined (cone) sheets and regional dykes. In Iceland, the inclined sheets form dense swarms of (mostly) basaltic, 0.5–1 m thick sheets, dipping either at 20–50° or at 75–90° towards the central volcano to which they belong. The regional dykes are (mostly) basaltic, 4–6 m thick, subvertical, subparallel and form swarms, less dense than those of the sheets but tens of kilometres long, in the parts of the volcanic systems that are outside the central volcanoes. In both types of swarms, the intrusion intensity decreases with altitude in the lava pile. Theoretical models generally indicate very high crack-tip stresses for propagating dykes and sheets. Nevertheless, most of these intrusions become arrested at various crustal depths and never reach the surface to supply magma to volcanic eruptions. Two principal mechanisms are proposed to explain arrest of dykes and sheets. One is the generation of stress barriers, that is, layers with local stresses unfavourable for the intrusion propagation. The other is mechanical anisotropy whereby sheet intrusions become arrested at discontinuities. Stress barriers may develop in several ways. First, analytical solutions for a homogeneous and isotropic crust show that the intensity of the tensile stress associated with a pressured magma chamber falls off rapidly with distance from the chamber. Thus, while dyke and sheet injection in the vicinity of a chamber may be favoured, dyke and sheet arrest is encouraged in layers (stress barriers) at a certain distance from the chamber. Second, boundary-element models for magma chambers in a mechanically layered crust indicate abrupt changes in tensile stresses between layers of contrasting Young’s moduli (stiffnesses). Thus, where soft pyroclastic layers alternate with stiff lava flows, as in many volcanoes, sheet and dyke arrest is encouraged. Abrupt changes in stiffness between layers are commonly associated with weak and partly open contacts and other discontinuities. It follows that stress barriers and discontinuities commonly operate together as mechanisms of dyke and sheet arrest in central volcanoes.     

The Pitcairn hotspot in the South Pacific: distribution and composition of submarine volcanic sequences

Multibeam bathymetry and bottom imaging (Simrad EM12D) studies on an area of about 9500 km² were conducted over the Pitcairn hotspot near 25°10′S, 129° 20′W. In addition, 15 dives with the Nautile submersible enabled us to obtain ground-true observations and to sample volcanic structures on the ancient ocean crust of the Farallon Plate at 3500–4300 m depths. More than 100 submarine volcanoes overprint the ancient crust and are divided according to their size into large (>2000 m in height), intermediate (500–2000 m high) and small (<500 m high) edifices. The interpretation of seafloor backscatter imagery accompanied by submersible observations and sampling enabled us to infer that the total volume of submarine lava erupted during hotspot activity is about 5900 km³ within a radius of about 110 km. The most recent volcanic activities occur on both small and large edifices composed of a great variety of lava flows. These flows vary in composition, following a succession from picritic basalt to alkali basalt, trachybasalt, trachy-andesite and to trachyte. Their large range of SiO₂ (48–62%), Na₂O+K₂O (2–11%), Ba (300–1300 ppm), MgO (1–11%), Nb (19–130 ppm), Ni (4–400 ppm) and rare earth elements suggests that crystal–liquid fractionation from basanite and/or picritic melt sources was a major process. The variation in composition between the least evolved basaltic rocks and the other more evolved silicic lava is marked by a difference in their flow morphology (pillow, giant tubes, tabular to blocky flows). The lava composition and field observation indicate that several magmatic pulses giving rise to cyclic eruptions are responsible for the construction of the edifices. The two larger edifices (>2000 m high) show more extensive eruptive events and a wider range in compositional variability than the smaller (<500 m high) ones. Several (five) submersible transects made along the slope of one of the largest edifices (Bounty) enabled us to observe at least nine successive eruptive cycles progressing from pillow and giant tubular basalt to tabular/blocky trachy-andesite and trachyte flows. Pyroclasts and hyaloclastites are often found with these eruptive sequences. The smaller edifices, forming individualized cones, are built mainly of evolved silicic (SiO₂>53%) flows consisting essentially of alternating sequences of trachy-andesite and trachyte. The distribution and composition of the small edifices suggest that they are the result of sub-crustal forceful magma injection and channeling supplied from reservoirs associated with the large volcanoes.     

Volcanoes in the pre-Columbian life,legend, and archaeology of Costa Rica (Central America)

Costa Rica is located geographically in the southern part of the Central American Volcanic Front, a zone where interaction between the Mesoamerican and South American cultures occurred in pre-Columbian times. Several volcanoes violently erupted during the Holocene, when the first nomadic human hunters and later settlers were present. Volcanic rocks were the most important geo-resource in making artifacts and as construction materials for pre-Columbian inhabitants. Some pottery products are believed to resemble smoking volcanoes, and the settlements around volcanoes would seem to indicate their influence on daily life. Undoubtedly, volcanic eruptions disrupted the life of early settlers, particularly in the vicinity of Arenal and Irazú volcanoes, where archaeological remains show transient effects and displacement caused by periodical eruptions, but later resilient occupations around the volcanoes. Most native languages are extinct, with the exception of those presently spoken in areas far away from active volcanoes, where no words are related to volcanic phenomena or structures. The preserved legends are ambiguous, suggesting that they were either produced during the early Spanish conquest or were altered following the pre-Columbian period.     

Spatial and temporal evolution of arc volcanism in the northeast Honshu and Izu‐Bonin Arcs: Evidence of small‐scale convection under the island arc?

Abstract   Arc volcanism of the past 10 my in the northeast Honshu and Izu-Bonin Arcs shows several notable features. In the northeast Honshu Arc, the spatial distribution of volcanism exhibits several clusters elongated nearly perpendicular to the arc and the possible migration of volcanism from the back-arc side to the volcanic front side, at least, during the past 5 my. The pattern of clusters seems to have flip-flopped around 5 Ma. In the Izu-Bonin Arc, there are a series of across-arc seamount chains, in which volcanic activity occurred from ca 17 Ma to ca 3 Ma, similar to the clusters of the northeast Honshu Arc, although the recent active rifting occurs almost parallel to the arc. On the basis of studies of numerical modeling, these features might be explained, at least qualitatively, by the small-scale convection under the island arc. Several inferences can be made from our modeling results for the tectonics of the Izu-Bonin Arc. The angle of dip of subducting plate in the Izu-Bonin Arc might have increased. This can explain the disappearance of volcanism along the seamount chains and the recent along-arc volcanism with narrow rifting. The trend of seamount chains, which is oblique to the arc, might not be their intrinsic feature but rather a result of the lateral movement of the back-arc region after their formation. These inferences can be tested by the future detailed morphological and chronological studies of the Izu-Bonin Arc.     

滇西地区壳幔解耦与腾冲火山区岩浆活动的深部构造研究

根据青藏东部边缘的深部地球物理资料,分析了滇西地区壳幔耦合和腾冲火山区岩浆活动的深部构造特征,确认了地幔各向异性与上地幔速度结构(包括P波速度和S波速度)的内在联系,指出产生这一结果的原因与以腾冲火山区为中心的地幔热物质上涌有关:上地幔顶部平均温度升高导致介质强度降低,在印支块体的侧向挤压或印缅块体的向东俯冲作用下发生韧性变形,造成滇西地区地幔各向异性的快波方向与青藏东部地壳块体的旋转方向不一致.此外,鉴于中下地壳低速层的横向非均匀性,估计韧性流动并非贯通青藏高原的东部边缘,而是被不同的构造块体和边界断裂限定在局部地区.总体而言,滇西地区下地壳的地震波速度和电阻率偏低,具备发生韧性变形的构造条件.作为地壳和上地幔之间的解耦层,它使得青藏东部地壳块体旋转产生的构造应力未能传输至上地幔.腾冲火山区的地壳结构与不同时期的岩浆活动有关,火山区东侧的高速结构代表了上新世时期火山通道内冷凝固结的岩浆侵入体或难以挥发的高密度残留物质,火山区西侧的低速结构反映了更新世以来持续至今的岩浆活动,壳内岩浆源主要分布在10～20km的深度范围内,横向尺度约为15～20km,有可能通过地壳深部的断裂与上地幔岩浆源区相连,估计腾冲火山区下方的岩浆活动将持续进行.     

Propagation of P and S-waves in magmas with different crystal contents: Insights into the crystallinity of magmatic reservoirs

Increasing amount of crystals tends to reduce the mobility of magmas and modifies its elastic characteristics (e.g. [Caricchi, L. et al., 2007. Non-Newtonian rheology of crystal-bearing magmas and implications for magma ascent dynamics. Earth and Planetary Science Letters, 264: 402–419.; Bagdassarov, N., Dingwell, D.B. and Webb, S.L., 1994. Viscoelasticity of crystal- and bubble-bearing rhyolite melts. Physics of the Earth and Planetary Interior, 83: 83–99.]). To quantify the effect of crystals on the elastic properties of magmas the propagation speed of shear and compressional waves have been measured at pressure and temperatures relevant for natural magmatic reservoirs. The measurements have been performed in aggregates at variable particle fractions (? = 0–0.7). The measurements were carried out at 200 MPa confining pressure and temperatures between 300 K and 1273 K (i.e. across the glass transition temperature (Tg) from glass to melt). The specimens were mixtures of a haplogranitic melt containing 5.25 wt.% H₂O and variable amounts of sub-spherical alumina particles. Additional experiments were carried out on a sample containing both, crystals and air bubbles. The temperature derivatives of the shear (dVs/dT) and compressional wave (dVp/dT) velocities for pure glass and samples with a crystal fraction of 0.5 are different below and above the glass transition temperature. For a crystal fraction 0.7, only dVp/dT changed above the Tg. In the presence of gas bubbles, Vp and Vs decrease constantly with increasing temperature. The bubble-bearing material yields a lower bulk modulus relative to its shear modulus. The propagation velocities of compressional and shear waves increase non-linearly with increasing crystal fraction with a prominent raise in the range 0.5 < ? < 0.7. The speed variations are only marginally related to the density increase due to the presence of crystals, but are dominantly related to the achievement of a continuous crystal framework. The experimental data set presented here can be utilized to estimate the relative proportions of crystals and melt present in a magmatic reservoir, which, in turn, is one of the fundamental parameters determining the mobility of magma and, consequently, exerting a prime control on the likelihood of an eruption from a sub-surficial magma reservoir.     

Gas geochemistry of the Valles caldera region, New Mexico and comparisons with gases at Yellowstone, Long Valley and other geothermal systems

Noncondensible gases from hot springs, fumaroles, and deep wells within the Valles caldera geothermal system (210–300°C) consist of roughly 98.5 mol% CO₂, 0.5 mol% H₂S, and 1 mol% other components. ³He/⁴He ratios indicate a deep magmatic source (R/R_a up to 6) whereas δ¹³C–CO₂ values (−3 to −5‰) do not discriminate between a mantle/magmatic source and a source from subjacent, hydrothermally altered Paleozoic carbonate rocks. Regional gases from sites within a 50-km radius beyond Valles caldera are relatively enriched in CO₂ and He, but depleted in H₂S compared to Valles gases. Regional gases have R/R_a values ≤1.2 due to more interaction with the crust and/or less contribution from the mantle. Carbon sources for regional CO₂ are varied. During 1982–1998, repeat analyses of gases from intracaldera sites at Sulphur Springs showed relatively constant CH₄, H₂, and H₂S contents. The only exception was gas from Footbath Spring (1987–1993), which experienced increases in these three components during drilling and testing of scientific wells VC-2a and VC-2b. Present-day Valles gases contain substantially less N₂ than fluid inclusion gases trapped in deep, early-stage, post-caldera vein minerals. This suggests that the long-lived Valles hydrothermal system (ca. 1 Myr) has depleted subsurface Paleozoic sedimentary rocks of nitrogen. When compared with gases from many other geothermal systems, Valles caldera gases are relatively enriched in He but depleted in CH₄, N₂ and Ar. In this respect, Valles gases resemble end-member hydrothermal and magmatic gases discharged at hot spots (Galapagos, Kilauea, and Yellowstone).     

In situ Permeability Measurements Based on a Radial Gas Advection Model: Relationships Between Soil Permeability and Diffuse CO2 Degassing in Volcanic Areas

In this paper we have developed a new method for measuring in situ soil permeability, which is based on the theory of radial gas advection through an isotropic porous medium. The method was tested in the laboratory and at several locations on the island of Vulcano (Aeolian Islands, Italy). It consists of a special device which generates a gas source at a depth of 50 cm and it permits measurement of the relative induced pressure in nearby soil at different depths. The characteristic error of the method was less than 10%. Furthermore, soil permeability measurements were carried out in the island of Vulcano during different periods of the year (between May 2000 and June 2001). A strong decrease in permeability in the upper layers of the soil during and after rainfall was noted, with very poor correlations between the spatial distributions of soil CO₂ flux and shallow soil permeability.     

Volcanoes as elastic inclusions: Their effects on the propagation of dykes,volcanic fissures,and volcanic zones in Iceland

Mechanically, many volcanoes may be regarded as elastic inclusions, either softer (with a lower Young's modulus) or stiffer (with a higher Young's modulus) than the host-rock matrix. For example, many central volcanoes (stratovolcanoes, composite volcanoes) are composed of rocks that are softer than the crustal segments that host them. This is particularly clear in Iceland where central volcanoes are mostly made of soft rocks such as rhyolite, pyroclastics, hyaloclastites, and sediments whereas the host rock is primarily stiff basaltic lava flows. Most active central volcanoes also contain fluid magma chambers, and many have collapse calderas. Fluid magma chambers are best modelled as cavities (in three dimensions) or holes (in two dimensions), entire calderas as holes, and the ring faults themselves, which commonly include soft materials such as breccias, as soft inclusions. Many hyaloclastite (basaltic breccias) mountains partly buried in the basaltic lava pile also function as soft inclusions. Modelling volcanoes as soft inclusions or holes, we present three main numerical results. The first, using the hole model, shows the mechanical interaction between all the active central volcanoes in Iceland and, in particular, those forming the two main clusters at the north and south end of the East Volcanic Zone (EVZ). The strong indication of mechanical interaction through shared dykes and faults in the northern cluster of the EVZ is supported by observations. The second model, using a soft inclusion, shows that the Torfajökull central volcano, which contains the largest active caldera in Iceland, suppresses the spreading-generated tensile stress in its surroundings. We propose that this partly explains why the proper rift zone northeast of Torfajökull has not managed to propagate through the volcano. Apparently, Torfajökull tends to slow down the rate of southwest propagation of the rift-zone part of the EVZ. The third model, again using a soft inclusion, indicates how the lateral propagation of a segment of the 1783 Laki fissure became arrested in the slopes of the hyaloclastite mountain Laki.     

Petrogenesis and tectonic setting of bimodal volcanism in the Sakoli Mobile Belt, Central Indian shield

The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO₂ shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial ¹⁴³Nd/¹⁴⁴Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNd_t (t = 2000 Ma) varies between ?0.19 and +2.22 for the basic volcanic rock and between ?2.85 and ?4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.     

Cenozoic volcanism and lithospheric tectonic evolution in Qiangtang area, northern Qinghai-Tibet Plateau

Accompanying with the shortening,thickening and uplifting of the lithosphere,a series of Cenozoic potassic volcanic rock zones are developed in the northern Qinghai-Tibet Plateau.From south to north,the volcanic rocks can be divided into three volcanicrock belts:Qiangtang-Nangqian volcanic belt,Middle Kunlun-Hoh Xil volcanic belt and Western Kunlun-Eastern Kunlun volcanic belt[1].Spatiotemporal evolu-tion of the volcanism and the origins of magmas con-strains on the pulsing uplifting and …     

Radionuclides and sulfur content in Mount Etna plume in 1983–1995: new constraints on the magma feeding system

Radionuclide activities (²¹⁰Pb, ²¹⁰Bi, ²¹⁰Po) were investigated in Mount Etna plume from 1983 to 1995. At SE crater the long-term observation (12 years) of the ²¹⁰Po/²¹⁰Pb ratio shows that it can behave as a degassing vent not directly related to the main magma reservoir depending on the magma level inside the volcano. Since 1992, the simultaneous determination of radionuclides and sulfur in the main plume results in new constraints on the degassing model of Lambert et al. (Earth Planet. Sci. Lett., 76 (1986) 185). The ²¹⁰Po/SO₂ and ²¹⁰Pb/SO₂ ratios enable us to identify two sources of ²¹⁰Po in the plume: one is magmatic, correlated with SO₂, the other one is an additional component issued from the decay of ²¹⁰Pb in the shallow degassing cell, and depends directly on the residence time of the gases before their emission. Estimations of the volume of degassing magma, the residence time of the gases and the proportion of undegassed magma renewing the shallow degassing cell are given for the period 1992–1995. During the 1992 eruption, the rate of degassing magma volume is estimated to have been as high as 5×10⁶ m³/day, and the volume of the shallow degassing magma reservoir about 0.5 km³. In 1994 and 1995 the rate of non-erupted degassing magma volume was estimated to have been about 0.18 km³/year. During the entire 1983–1995 period, only 15–20% of the degassed magma was erupted.     

Hydrodynamics and sediment-transport in the nearshore of Poverty Bay, New Zealand: Observations of nearshore sediment segregation and oceanic storms

Nearshore regions act as an interface between the terrestrial environment and deeper waters. As such, they play important roles in the dispersal of fluvial sediment and the transport of sand to and from the shoreline. This study focused on the nearshore of Poverty Bay, New Zealand, and the processes controlling the dispersal of sediment from the main source, the Waipaoa River. Hydrodynamics and sediment-transport in water shallower than 15 m were observed from April through mid-September 2006. This deployment afforded observations during 3-4 periods of elevated river discharge and 5 dry storms.Similar wind, river discharge, wave, current, and turbidity patterns were characterized during three of the wet storms. At the beginning of each event, winds blew shoreward, increasing wave heights to 2-3 m within Poverty Bay. As the cyclonic storms moved through the system the winds reversed direction and became seaward, reducing the local wave height and orbital velocity while river discharge remained elevated. At these times, high river discharge and relatively small waves enabled fluvially derived suspended sediment to deposit in shallow water. Altimetry measurements indicated that at least 7 cm was deposited at a 15 m deep site during a single discharge event. Turbidity and seabed observations showed this deposition to be removed, however, as large swell waves from the Southern Ocean triggered resuspension of the material within three weeks of deposition. Consequently, two periods of dispersal were associated with each discharge pulse, one coinciding with fluvial delivery, and a second driven by wave resuspension a few weeks later. These observations of nearfield sediment deposition contradict current hypotheses of very limited sediment deposition in shallow water offshore of small mountainous rivers when floods and high-energy, large wave and fast current, oceanic conditions coincide.Consistently shoreward near-bed currents, observed along the 10 m isobath of Poverty Bay, were attributed to a combination of estuarine circulation, Stokes drift, and wind driven upwelling. Velocities measured at the 15 m isobath, however, were directed more alongshore and diverged from those at the 10 m isobath. The divergence in the currents observed at the 10 and 15 m locations seemed to facilitate segregation of coarse and fine sediment, with sand transported near-bed toward the beach, while suspended silts and clays were exported to deeper water.     

Measurement and Result of Soil Gas Radon and Soil Gas Mercury in the Exploration of Haihe Hidden Fault

In the exploration of the hidden Haihe fault, radon and mercury in soil gas were measured by using FG-3017 radon detector and XG-4 mercury analyzer. In this paper, based on the measurement results of 12 fault gas profiles, and integrating with the exploration results of artificial seismic prospecting, the relationship between anomalous site of fault gas and fault location is analyzed. Using the relationship between anomalous strength of fault gas and fault activity, the activity of Haihe fault is studied, thus the location and activity segmentation of the Haihe fault in Tianjin region are presented. This study shows that the method of fault gas detection can not only identify the preliminary location of fault, but also make preliminary segmentation of fault activity. The fault detected by the method of fault gas measurement is shown as a band. Through contrasting with exploration results of artificial seismic prospecting and analyzing, we find that the fault is located inside the band. According to the measurements of soil gas radon, the Haihe fault can be divided into east and west segments and the activity of the east segment of Haihe fault is stronger than that of the west segment. This is only a relative result, and it is difficult to judge whether the fault is active or not with this result.