Chemical Characterisation of Natural Ilmenite: A Possible New Reference Material     

Patrick H. Donohue  Antonio Simonetti  Clive R. Neal 《Geostandards and Geoanalytical Research》2012,36(1):61-73

Seven ilmenite (FeTiO₃) megacrysts derived from alnöite pipes (Island of Malaita, Solomon Islands) were characterised for their major and trace element compositions in relation to their potential use as secondary reference materials for in situ microanalysis. Abundances of thirteen trace elements obtained by laser ablation ICP‐MS analyses (using the NIST SRM 610 glass reference material) were compared with those determined by solution‐mode ICP‐MS measurements, and these indicated good agreement for most elements. The accuracy of the LA‐ICP‐MS protocol employed here was also assessed by repeated analysis of MPI‐DING international glass reference materials ML3B‐G and KL2‐G. Several of the Malaitan ilmenite megacrysts exhibited discrepancies between laser ablation and solution‐mode ICP‐MS analyses, primarily attributed to the presence of a titano‐magnetite exsolution phase (at the grain boundaries), which were incorporated solely in the solution‐mode runs. Element abundances obtained by LA‐ICP‐MS for three of the ilmenite megacrysts (CRN63E, CRN63H and CRN63K) investigated here had RSD (2s) values of < 20% and therefore can be considered as working values for reference purposes during routine LA‐ICP‐MS analyses of ilmenite.  相似文献   

Cone morphologies associated with shallow marine eruptions: east Pico Island, Azores     

Neil C. Mitchell  Rachelle Stretch  Clive Oppenheimer  Daniel Kay  Christoph Beier 《Bulletin of Volcanology》2012,74(10):2289-2301

Eruptions in shallow water typically produce cones of volcaniclastic material. In order to identify any systematic effects of water depth and other environmental parameters on cone morphology, we have measured the heights and widths of cones in multibeam echo-sounder data from a submarine ridge extending southeast from Pico Island, Azores. XRF analyses of dredged samples show that lavas here vary compositionally from alkali basalt to trachybasalt and trachyandesite. Cones in deeper water are generally steep-sided with upper flanks close to 30°, the dip of talus at the angle of repose. However, height/width ratios of cones vary more in shallow water (200?C400-m summit depth) with extreme values below 0.1; while some shallow-water cones are steep-sided as in deep water, others are much flatter. Three such cones lie on a bench at 300-m depth immediately east of Pico Island and have flank slopes of only 10?C20°. We speculate that exceptionally shallow cone slopes here were produced by forced spreading of the erupting columns on reaching the water?Cair density barrier.  相似文献   

Social studies of volcanology: knowledge generation and expert advice on active volcanoes   总被引：2，自引：1，他引：1  

Amy Donovan  Clive Oppenheimer  Michael Bravo 《Bulletin of Volcanology》2012,74(3):677-689

This paper examines the philosophy and evolution of volcanological science in recent years, particularly in relation to the growth of volcanic hazard and risk science. It uses the lens of Science and Technology Studies to examine the ways in which knowledge generation is controlled and directed by social forces, particularly during eruptions, which constitute landmarks in the development of new technologies and models. It also presents data from a survey of volcanologists carried out during late 2008 and early 2009. These data concern the felt purpose of the science according to the volcanologists who participated and their impressions of the most important eruptions in historical time. It demonstrates that volcanologists are motivated both by the academic science environment and by a social concern for managing the impact of volcanic hazards on populations. Also discussed are the eruptions that have most influenced the discipline and the role of scientists in policymaking on active volcanoes. Expertise in volcanology can become the primary driver of public policy very suddenly when a volcano erupts, placing immense pressure on volcanologists. In response, the epistemological foundations of volcanology are on the move, with an increasing volume of research into risk assessment and management. This requires new, integrated methodologies for knowledge collection that transcend scientific disciplinary boundaries.  相似文献   

On the time-scale of thermal cycles associated with open-vent degassing   总被引：1，自引：1，他引：0  

L. Spampinato  C. Oppenheimer  A. Cannata  P. Montalto  G. G. Salerno  S. Calvari 《Bulletin of Volcanology》2012,74(6):1281-1292

Thermal imagery obtained with portable infrared cameras is widely used to track and measure volcanic phenomena. In the case of explosive eruptions, both air and ground-based thermal monitoring have enabled collection of data streams from relatively safe distances. Analysis of these data have enabled the characterisation of different explosive regimes, parameterisation of eruptive plumes, and assessment on the dynamics occurring in the shallow system. Here we explore the suitability of infrared imagers for investigating the short time scale eruptive behaviour of three basaltic volcanoes. We present high-time resolution thermal image data-sets recorded at Etna, Stromboli and Kīlauea volcano. At the time of observations, all three exhibited pulsed degassing. Signal processing of the mean apparent temperature time-series highlights four broad classes of cyclic temperature changes at the three volcanoes based on characteristic time-scales revealed in the periodograms: (1) <15?s, (2) ~20-50?s, (3) ~1-10?min, and (4) 12–90?min. Based on previous studies and integrating time-series results with qualitative visible and thermal observations and, in case of Kīlauea, also with SO₂ column amounts in the plume, we hypothesise that short cycles relate mainly to bursting of overpressured gas bubbles at the magma surface, while long cycles might be associated with mechanisms of gas slug formation and ascent, and to the emplacement and drainage of a lava lake. At Kīlauea, slow fluctuations may reflect periodic variations of the lava lake surface level. The data from all three volcanoes reveal superimposition of degassing cycles of different frequencies, suggesting link through common magmatic processes and physical properties.  相似文献   

Photosynthesis of Anabaena in Lake Rotongaio: Short‐term responses to a changing light environment     

Richard D. Robarts  Clive Howard‐Williams 《新西兰海洋与淡水研究杂志》2013,47(3)

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Remarkable geochemical changes and degassing at Voui crater lake,Ambae volcano,Vanuatu     

Philipson Bani  Clive Oppenheimer  Johan C. Varekamp  Thomas Quinou  Michel Lardy  Simon Carn 《Journal of Volcanology and Geothermal Research》2009

Ambae (also known as Aoba), is a 38 × 16 km² lozenge-shaped island volcano with a coastal population of around 10 000. At the summit of the volcano is lake Voui — one of the largest active crater lakes worldwide, with 40 × 10⁶ m³ of acidic water perched 1400 m a.s.l. After more than 300 years of dormancy, Ambae volcano reawakened with phreatic eruptions through Voui in 1995, and culminating in a series of surtseyan eruptions in 2005, followed by a rapid and spectacular colour change of the lake from light blue to red in 2006. Integrating lake water chemistry with new measurements of SO₂ emissions from the volcano during the 2005–2006 eruptive period helps to explain the unusual and spectacular volcanic activity of Ambae — initially, a degassed magma approached the lake bed and triggered the surtseyan eruption. Depressurization of the conduit facilitated ascent of volatile-rich magma from the deeper plumbing system. The construction of a cone during eruption and the high degassing destabilised the equilibrium of lake stratification leading to a limnic event and subsequently the spectacular colour change.  相似文献   

A new laboratory technique for determining the compressional wave properties of marine sediments at sonic frequencies and in situ pressures     

Clive McCann  Jeremy Sothcott  Angus I. Best 《Geophysical Prospecting》2014,62(1):97-116

We describe a new laboratory technique for measuring the compressional wave velocity and attenuation of jacketed samples of unconsolidated marine sediments within the acoustic (sonic) frequency range 1–10 kHz and at elevated differential (confining – pore) pressures up to 2.413 MPa (350 psi). The method is particularly well suited to attenuation studies because the large sample length (up to 0.6 m long, diameter 0.069 m) is equivalent to about one wavelength, thus giving representative bulk values for heterogeneous samples. Placing a sediment sample in a water‐filled, thick‐walled, stainless steel Pulse Tube causes the spectrum of a broadband acoustic pulse to be modified into a decaying series of maxima and minima, from which the Stoneley and compressional wave, velocity and attenuation of the sample can be determined. Experiments show that PVC and copper jackets have a negligible effect on the measured values of sediment velocity and attenuation, which are accurate to better than ± 1.5% for velocity and up to ± 5% for attenuation. Pulse Tube velocity and attenuation values for sand and silty‐clay samples agree well with published data for similar sediments, adjusted for pressure, temperature, salinity and frequency using standard equations. Attenuation in sand decreases with pressure to small values below Q^?1 = 0.01 (Q greater than 100) for differential pressures over 1.5 MPa, equivalent to sub‐seafloor depths of about 150 m. By contrast, attenuation in silty clay shows little pressure dependence and intermediate Q^?1 values between 0.0206–0.0235 (Q = 49–43). The attenuation results fill a notable gap in the grain size range of published data sets. Overall, we show that the Pulse Tube method gives reliable acoustic velocity and attenuation results for typical marine sediments.  相似文献   

Secondary Ion Mass Spectrometry Methodology for Isotopic Ratio Measurement of Micro‐Grains in Thin Sections: True Grain Size Estimation and Deconvolution of Inter‐Grain Size Gradients and Intra‐Grain Radial Gradients     

Clive Jones  David A. Fike  Katja M. Meyer 《Geostandards and Geoanalytical Research》2019,43(1):61-76

In a thin section, grains that were approximately spherical in situ appear circular in cross section, and the distribution of apparent diameters frequently assumed to be their size distribution. Scanning ion imaging by secondary ion mass spectrometry (SIMS) is capable of providing precise (< 1‰) stable isotope ratio measurements of such grains, but, importantly, also registers their rate of evolution in apparent size as they are ablated by the primary beam. By assessing rates of radius change with depth, the described methodology enables the ‘true’ size of grains to be estimated, as well as the distance of the sectioned surface from the original grain centre. Transects in three dimensions are made possible, and this capability enables better identification (and thus separation) of both inter‐grain chemical signatures as a function of grain size, and intra‐grain radial trends. In this example, we highlight the specific application to pyrite (FeS₂) minerals, which are frequently analysed by SIMS to determine their inter‐grain and intra‐grain geochemical variations, particularly in their sulfur stable isotopic ratios (δ³⁴S). Benefits of the new methodology over the Faraday cup ‘spot mode’ are described. Data correction algorithms and precision considerations are discussed.  相似文献   

Probing the magma plumbing of Erebus volcano,Antarctica, by open-path FTIR spectroscopy of gas emissions     

Clive Oppenheimer  Philip R. Kyle 《Journal of Volcanology and Geothermal Research》2008

Gas emissions from Erebus volcano, Antarctica, were measured by open-path Fourier transform infrared spectroscopy to understand degassing of its magmatic system. Two degassing phonolite lava lakes were present in the summit crater during observation in December 2004. We report analyses of H₂O, CO₂, CO, SO₂, HF, HCl and OCS, (in order of molar abundance) in the plumes. Variations in the proportions of these species strongly reflect the dynamics of degassing, and sourcing of gas from different depths in the magmatic network. The highest observed ratios of CO₂ and H₂O are consistent with gas extracted from the melt at a depth of up to ∼ 2 km below the lava lakes. Magma degassing above this depth contributes to a higher H₂O/CO₂ proportion in the airborne plume. The ratio therefore reflects the balance of deeper vs. shallower contributions of volatiles and, possibly, a combination of closed- and open-system degassing. We observe a strong contrast in HF content in emissions from the two lava lakes, which we attribute to differing levels of magma ascent and/or cooling and crystallization of the magma supply. Fluxes of all gas species were determined using independent SO₂ flux determinations and measured gas ratios. In the case of CO₂ and water, ∼ 1 and ∼ 0.4 m³ s^− 1, respectively, of parental basanite magma are required to sustain the calculated output. The discrepancy between the two figures is readily explained by sequestration of part of the magma supply at depth such that it only partially degasses its complement of water.  相似文献   

Attenuation of P- and S-waves in limestones   总被引：2，自引：1，他引：1  

Solomon Assefa  Clive McCann  & Jeremy Sothcott 《Geophysical Prospecting》1999,47(3):359-392

Ultrasonic compressional- and shear-wave attenuation measurements have been made on 40, centimetre-sized samples of water- and oil-saturated oolitic limestones at 50 MPa effective hydrostatic pressure (confining pressure minus pore-fluid pressure) at frequencies of about 0.85 MHz and 0.7 MHz respectively, using the pulse-echo method. The mineralogy, porosity, permeability and the distribution of the pore types of each sample were determined using a combination of optical and scanning electron microscopy, a helium porosimeter and a nitrogen permeameter. The limestones contain a complex porosity system consisting of interparticle macropores (dimensions up to 300 microns) and micropores (dimensions 5–10 microns) within the ooids, the calcite cement and the mud matrix. Ultrasonic attenuation reaches a maximum value in those limestones in which the dual porosity system is most fully developed, indicating that the squirt-flow mechanism, which has previously been shown to occur in shaley sandstones, also operates in the limestones. It is argued that the larger-scale dual porosity systems present in limestones in situ could similarly cause seismic attenuation at the frequencies of field seismic surveys through the operation of the squirt-flow mechanism.  相似文献