bluuebunny/crossref_metadata_2025_split huggingface dataset:
Water and halogens in volcanic clasts: tracers of degassing processes during Plinian and dome-building eruptions
Authors: Villemant B. , Boudon G. , Nougrigat S. , Poteaux S. , Michel A. 
Published: April 2008

Abstract

Magma degassing may occur either with no significant gas escape from the magma column, which corresponds with typical Plinian type eruptions, or with gas loss, which corresponds with typical effusive (dome-building) eruptions. Magma degassing may also lead to melt crystallization, which modifies the residual melt composition and, in turn, may significantly affect the degassing conditions. We propose a method for modelling these processes for H
2
O-rich rhyolitic melts through measurements of volatiles (H
2
O, Cl, Br) in the microcrystalline matrix and glass of erupted volcanic clasts (pumice and dome clasts). This method is applied to two Plinian and dome-building eruptions at Mount Pelée (Martinique) and Santa Maria (Guatemala) volcanoes. Extreme magma degassing and crystallization during dome-building eruptions may explain the contrasts in halogen and H
2
O contents of residual melts of dense volcanic clasts: they display very large ranges of Cl contents (few ppm to thousands of ppm), whereas the ranges of H
2
O contents are much narrower, and lower than 1%. This method allows prediction of the evolution of volcanic gas chemistry (as HCl content or HCl/HF ratio) as a function of the degassing style of magma at shallow depth.
Magma extrusion dynamics revealed by high-frequency gas monitoring at Soufrière Hills volcano, Montserrat
Authors: Young S. R. , Voight B. , Duffell H. J. 
Published: April 2008

Abstract

Andesitic to dacitic dome-building volcanoes often present a problem for eruption forecasting because signs of impending activity can be minimal or ambiguous. Gas monitoring is one of a number of techniques used to assist in eruption forecasting. However, a variety of explanations have been offered for the large variations in gas release that are commonly reported from erupting volcanoes. Difficulties in interpretation can arise because gas-flux measurements are generally acquired at lower sampling rates than other geophysical observations. Here, we report SO
2
flux measurements, by correlation spectroscopy, recorded semi-continuously during December 1999 to January 2000 at the Soufrière Hills Volcano, Montserrat. We compare these data to continuously recorded seismic records, and interpret the results in terms of conduit dynamics. We demonstrate two- to six-fold variations in gas flux over a few hours, and show that these variations can be systematic and directly correlated with long-period swarm seismicity. For the period of study, we find that the gas-flux peak lags several tens of minutes behind the peak in seismic energy release. These features are consistent with models of oscillating magma flow, where magma viscosity is dependent on melt volatile content. We propose that seismicity reflects conduit pressurization, and find that gas flux directly reflects magma flow rate. Although other volcanoes might behave differently, our results suggest that it can be possible to use continuous gas measurements to monitor conduit behaviour, perhaps providing short-term warnings of impending eruptions.
The tropospheric sulphur cycle and the role of volcanic SO
2
Authors: Stevenson D. S. , Johnson C. E. , Collins W. J. , Derwent R. G. 
Published: April 2008

Abstract

A global three-dimensional chemistry-transport model has been applied to study the tropospheric sulphur cycle, and in particular the volcanic component. The model is in general agreement with previous studies of the global S budget. We find that volcanic emissions constitute 10% of the present-day global SO
2
source to the atmosphere, but form 26% of the SO
2
burden, and 14% of the sulphate aerosol burden. Two previous modelling studies suggested that the volcanic fraction of sulphate was 18% and 35%, from sources representing 7% and 14%, respectively, of the global total SO
2
emission. The results are dependent upon various assumptions about volcanic emissions (magnitude, geographical location, altitude), the global distribution of oxidants, and the physical processes of dry and wet deposition. Because of this dependence upon poorly constrained parameters, it is unclear which modelling study is closest to the truth.
Compositional variation in tropospheric volcanic gas plumes: evidence from ground-based remote sensing
Authors: Horrocks L. A. , Oppenheimer C. , Burton M. R. , Duffell H. J. 
Published: April 2008

Abstract

Remotely sensed measurements of volcanic plumes have been undertaken for 30 years with instruments such as the correlation spectrometer, and more recently, open-path Fourier transform infrared (OP-FTIR) spectrometers. Observations are typically made several kilometres from the source, by which time chemical reactions may have occurred in the plume, overprinting the source composition and flux. Volcanological interpretations of such data therefore demand an understanding of the atmospheric processes initiated as gases leave the volcanic vent. Ground-based remote sensing techniques offer the temporal resolution, repeatability and quantitative analysis necessary for investigation of these processes. Here we report OP-FTIR spectroscopic measurements of gas emissions from Masaya Volcano, Nicaragua, between 1998 and 2001, and examine the influence of atmospheric processes on its tropospheric plume. Comparisons of observations made at the summit and down-wind, and in different measurement modes confirm that tropospheric processes and local meteorology have only minor impact on gas composition after the plume has left the crater. This study demonstrates that plume monitoring downwind provides a reliable proxy for at-crater sampling, and that volcanological information content is not obscured by the intervening transport. From February 1998 to May 2000, Masaya’s plume composition was strikingly stable and characterized by SO
2
/HCl and HCl/HF molar ratios of 1.6 and 5.0, respectively. Departures from this stable background composition are likely to signify changes in the volcanic system or degassing regime, as identified in April–May 2001.
Correlations between eruption magnitude, SO
2
yield, and surface cooling
Authors: Blake S. 
Published: April 2008

Abstract

Sulphurous gases from explosive eruptions have the potential to form stratospheric aerosols and so produce surface cooling on a hemispheric to global scale. However, testing for any correlation between SO
2
yield and surface cooling is hampered by instrumental SO
2
and temperature measurements being available for time periods that include only a few large eruptions. To overcome this, published dendroclimatological data, satellite (Total Ozone Mapping Spectrometer) data on SO
2
emissions, stratospheric optical depth data, and volcanological observations are integrated, revealing several relevant new correlations. First, the efficient conversion of SO
2
into stratospheric aerosols occurs when the ratio of plume height to tropopause height is greater than about 1.5. Second, the mass of emitted SO
2
correlates well with the mass of erupted magma. The SO
2
yield is 0.1 to 1% by mass of magma, irrespective of composition. The best-fit power law (
r
2
=0.67) is mass of SO
2
in Mt=1.77(mass of magma in Gt)
0.64
. Third, of the eruption clouds that are believed to have entered the stratosphere in the period 1400–1994, those with masses <5 Gt magma (DRE <2 km
3
) appear to have had insignificant effects on Northern Hemisphere summer temperature. The scattered data for eruptions of >10 Gt (>4 km
3
) magma suggest a mean cooling effect of about 0.35 °C.
Coupled mineral-fluid evolution of a basin and high: kaolinization in the SW England granites in relation to the development of the Plymouth Basin
Authors: Psyrillos Agamemnon , Burley Stuart D. , Manning David A. C. , Fallick Anthony E. 
Published: April 2008

Abstract
A new genetic model is proposed for the formation of the St Austell kaolin deposits, incorporating geological, isotopic, paragenetic and microthermometric evidence from kaolin-quartz veins combined with a reconstruction of the thermal evolution of the Cornubian pluton during the Mesozoic. Fluid inclusions in quartz, paragenetically associated with kaolin, document that the kaolinization took place at temperatures between 50°C and 100°C, indicating that the kaolinization is a low-temperature hydrothermal event coincident with the oil generation window.
Kaolinization occurred prior to the unroofing of the pluton, during the Late Jurassic to Early Cretaceous. The kaolinization is thus contemporary with the major Early Cretaceous uplift that affected the Cornubian massif as a consequence of rifting in the offshore Western Approaches. Geological, isotopic and geochemical considerations argue strongly against the involvement of unmodified meteoric waters in the kaolinization process. The most plausible fluid types for the kaolinization are either basinal brines expelled from Permo-Triassic sediments of the adjacent offshore Plymouth Basin, or highly evolved meteoric waters that circulated through the sediments enclosing the pluton. The kaolinization process converted large volumes of fractured granite to a porous quartz-kaolin rock matrix.
Applications of high-resolution X-ray computed tomography in petrology, meteoritics and palaeontology
Authors: Carlson W. D. , Rowe T. , Ketcham R. A. , Colbert M. W. 
Published: April 2008

Abstract
High-resolution and ultra-high-resolution X-ray computed tomography are rapid, non-destructive and extremely powerful techniques for three-dimensional examination and measurement of a great variety of geological materials and specimens with sizes from several millimetres to several decimetres. A review of recent applications in petrology, meteoritics and palaeontology, which utilized an instrument optimized for geological studies (High-Resolution X-ray Computed Tomography Facility of the University of Texas at Austin), documents an abundance of novel scientific results and illuminates the potential for still broader application of these techniques in the earth sciences.
Porosity measurements of sedimentary rocks by means of microfocus X-ray computed tomography (μCT)
Authors: Van Geet M. , Lagrou D. , Swennen R. 
Published: April 2008

Abstract
Porosity of reservoir rocks is an important petrophysical characteristic, used as a basic parameter in simulation studies for predicting reservoir quality. An extensive debate continues about the techniques that are available for porosity measurements and visualization. One aspect is the fact that petrophysical measurements are performed on volumetric samples, whereas classical geological petrography using a petrographical microscope is restricted to 2D analysis. This leads to a discrepancy between petrographical and petrophysical studies. This paper aims to evaluate microfocus X-ray computed tomography (μCT) as a technique that can link petrography and petrophysics. A short overview of the μCT technique is given, together with a discussion of its limitations, mainly due to artefacts. Optimization of image quality and procedures for quantification are outlined. μCT results for porosity measurements of a limestone and a sandstone are compared with results obtained by other techniques.
Geostatistics and the representative elementary volume of gamma ray tomography attenuation in rock cores
Authors: Vogel J. R. , Brown G. O. 
Published: April 2008

Abstract
Semivariograms of samples of Culebra Dolomite have been determined at two different resolutions for gamma ray computed tomography images. By fitting models to semivariograms, small-scale and large-scale correlation lengths are determined for four samples. Different semivariogram parameters were found for adjacent cores at both resolutions. Relative elementary volume (REV) concepts are related to the stationarity of the sample. A scale disparity factor is defined and is used to determine sample size required for ergodic stationarity with a specified correlation length. This allows for comparison of geostatistical measures and representative elementary volumes. The modifiable areal unit problem is also addressed and used to determine resolution effects on correlation lengths. By changing resolution, a range of correlation lengths can be determined for the same sample. Comparison of voxel volume to the best-fit model correlation length of a single sample at different resolutions reveals a linear scaling effect. Using this relationship, the range of the point value semivariogram is determined. This is the range approached as the voxel size goes to zero. Finally, these results are compared to the regularization theory of point variables for borehole cores and are found to be a better fit for predicting the volume-averaged range.
3D soil image characterization applied to hydraulic properties computation
Authors: Delerue J. F. , Perrier E. , Timmerman A. , Swennen R. 
Published: April 2008

Abstract
We propose a novel method to characterize the fluid-filled (usually air or water) space in images of porous media at the pore scale. First, an aperture map is created based on a skeleton process, to describe all local sizes in the pore space. Then the pore space is segmented in pores, defined as elementary objects that compose the pore space. Using this segmented image, a pore network is created, which is a graphic representation of the pore space that includes local sizes and direct information about connectivity at the pore scale. As an application of this method for pore space modelling, the equivalent hydraulic conductivity or permeability for a soil sample is computed.
Numerical modelling of reverse-density structures in soft non-Newtonian sediments
Authors: Harrison Philip , Maltman Alex J. 
Published: April 2008

Abstract
A numerical code has been used to simulate the flow patterns in geological soft sediments that are driven by buoyancy forces resulting from reverse-density stratification. The aim was to provide a clearer understanding of the different roles of initiating conditions, inertia and rheological behaviour on the morphologies and timing of formation of natural features such as load casts and flame structures. Particular attention was paid to the cuspate form of rising intrusions that is commonly seen in nature but that has proved elusive in most earlier experiments. The numerical results demonstrate that large localised initiating perturbations and inertial influence during flow both tend to cause a decrease in the wavelength of the resulting flow pattern and can, under certain circumstances, serve to promote a cuspate morphology. The use of a relatively low viscosity Newtonian fluid as an approximation of the coarse-grained upper layer coupled with, critically, power-law behaviour in the underlying clayey layer was also found to promote a cuspate form in the rising intrusion.
Near-surface sediment mobilization and methane venting in relation to hydrate destabilization in Southern Lake Baikal, Siberia
Authors: Van Rensbergen Pieter , Poort Jeffrey , Kipfer Rolf , de Batist Marc , Vanneste Maarten , Klerkx Jean , Granin Nick , Khlystov Oleg , Krinitsky Petr 
Published: April 2008

Abstract

Four seeps and mud extrusion features at the lake floor were discovered in August 1999 in the gas hydrate area in Lake Baikal’s South Basin. This paper describes these features in detail using side-scan sonar, detailed bathymetry, measurements of near-bottom water properties, selected seismic profiles and heat flow data calculated from the depth of the hydrate layer as well as obtained from
in situ
thermoprobe measurements. The interpretation of these data is integrated with published geochemical data from shallow cores. The seeps are identified as methane seeps and appear as mud cones (maximum 24 m high, 800 m in diameter) or low-relief craters (maximum 8 m deep, 500 m in diameter) at the lake floor. Mud cones (estimated to be approximately 50–100 ka old) appear to be older than the craters and have a different structural setting. Mud cones occur at the crest of rollover structures, in the footwall of a secondary normal fault, while the craters occur at fault splays. The seeps are found in an area of high heat flow where the base of the gas hydrate layer shallows rapidly towards the vent sites from about 400 m to about 160 m below the lake floor. At the site of the seep, a vertical seismic chimney disrupts the sedimentary stratification from the base of the hydrate layer to the lake floor. Integration of these results leads to the interpretation that focused destabilization of gas hydrate caused massive methane release and forced mud extrusion at the lake floor and that the gas seeps and mud diapirs in Lake Baikal do not have a deep origin. This is the first time that methane seeps and/or mud volcanoes associated with gas hydrate decomposition have been observed in a sub-lacustrine setting. The finding suggests that gas hydrate destabilization can create large pore fluid overpressures in the shallow subsurface (<500 m subsurface) and cause mud extrusion at the sediment surface.
Pore pressure/stress coupling and its implications for rock failure
Authors: Hillis Richard R. 
Published: April 2008

Abstract

Clastic dykes and sills witness that subsurface sediment mobilization is often controlled by the brittle failure of units ‘sealing’ overpressured and liquidized sediments. Brittle failure also imposes a limit on the buoyancy pressure that can be exerted by hydrocarbon columns. Conventional understanding of brittle failure induced by increasing pore pressure (P
p
) assumes that total minimum horizontal stress (σ
h
) is unaffected by changes in pore pressure. However, total minimum horizontal stress increases from shallow, normally pressured sequences to deeper, overpressured sequences. Data from the Canadian Scotian Shelf, the North Sea and the Australian North West Shelf demonstrate such P
p
/σ
h
coupling, with the minimum horizontal stress increasing at approximately 60–80% of the rate of pore pressure (i.e., Δσ
h
/ΔP
p
= 0.6–0.8). Hence, a greater increase in pore pressure can be sustained prior to brittle failure of units sealing overpressured compartments than would be predicted by conventional, uncoupled failure models. Furthermore, because total vertical stress is not similarly coupled to pore pressure, differential stress (σ
1
–σ
3
) reduces as pore pressure increases in normal fault regime basins. Thus, the mode of rock failure can not be inferred from differential stress in the stable state and P
p
/σ
h
coupling promotes tensile over shear failure.
New morphological observations on Triassic pterosaurs
Authors: Dalla Vecchia Fabio M. 
Published: April 2008

Abstract

Since 1973, about 20 specimens of Triassic pterosaurs have been found in northern Italy, Austria and Greenland, belonging to
Eudimorphodon, Peteinosaurus, Preondactylus
and
Austriadactylus
. Their age is middle to late Norian and
Eudimorphodon
is the most common genus. The restudy of the specimens shows that
Peteinosaurus
presents trimorphodonty in the lower jaw and a fibula unreduced in length, distally expanded and fused to the tibiotarsus above the lateral condyle. Specimen MCSNB 3359 does not show diagnostic features of
Peteinosaurus
and is referred to it with doubt, whereas MCSNB 3496 is not
Eudimorphodon
but
Peteinosaurus. Preondactylus, Peteinosaurus, ?Peteinosaurus
and
Dimorphodon
could form a monophyletic group.

The tarsus of Triassic pterosaurs consists of two proximal tarsals, which fuse to the tibia during ontogeny, forming a tibiotarsus, and two distal tarsals. The larger of the two proximal tarsals was probably the calcaneum. The lateral condyle of the tibiotarsus is larger or more well formed than the medial one. The shape of the distal tarsals is similar to that of the distal tarsals in
Dimorphodon
. The metatarsals did not spread and the foot was ectaxonic; metacarpal length increases from metacarpal I to IV. This suggests that footprints of Triassic pterosaurs were different from
Pteraichnus
-like footprints.

Some features are unique to Triassic pterosaurs. and
Austriadactylus
have a multicuspid dentition,
Peteinosaurus
has cuspules in the distal teeth of the lower jaw and
Preondactylus
has serrated maxillary teeth. This could be a convergent feature or symplesiomorphic for their clade. and
Austriadactylus
have very large maxillary teeth below the ascending process of the maxilla. and possibly
Preondactylus
do not have the bundles of elongated caudal zygapophyseal and haemapophyseal processes which are present in
Peteinosaurus
and in the Jurassic long-tailed pterosaurs.
A new crested ornithocheirid from the Lower Cretaceous of northeastern Brazil and the unusual death of an unusual pterosaur
Authors: Frey Eberhard , Martill David M. , Buchy Marie-Céline 
Published: April 2008

Abstract
An exceptionally well-preserved cranium and mandible of a new species of pterodactyloid pterosaur from the Nova Olinda Member of the Crato Formation (Aptian, Early Cretaceious) of the Araripe Basin, northeastern Brazil, is described. The new taxon is characterized by the presence of a caudally directed parietal crest similar to that seen in pteranodontids, but is referred to the Ornithocheiridae of the Ornithocheiroidea. The specimen is referred to a new genus within the Ornithocheiridae, as it lacks the diagnostic rostral crest and instead possesses this parietal crest oriented. A lanceolate leaf with frayed distal end wedged between the mandibular rami suggests the cause of death for the specimen.
Pterosaur (Pteranodontoidea, Pterodactyloidea) scapulocoracoid from the Early Cretaceous of Venezuela
Authors: Kellner Alexander W.A. , Moody John M. 
Published: April 2008

Abstract

The discovery of a left scapula and coracoid (MBLUZ P-911) representing the first evidence of a pterosaur from Venezuela is reported here. The material comes from the Lower Cretaceous (Aptian) Apón Formation, in the northwestern part of the country. In MBLUZ P-911 the scapula is significantly smaller than the coracoid, a synapomorphy of the Pteranodontoidea, according to Kellner. The coracoid of the Venezuelan specimen is more elongated and gracile than those of
Istiodactylus
and
Pteranodon
, and also lacks the ventromedial coracoidal flange present in the latter. Overall MBLUZ P-911 is very similar to the scapulocoracoid of the Anhangueridae, including the presence of a longitudinal ridge on the medial surface of the coracoid and a comparatively short scapula, and is therefore tentatively referred to this taxon. This occurrence extends the pterosaur record to the northern part of the South American portion of Gondwana.