Generation of Icelandic rhyolites: silicic lavas from the Torfajökull central volcano     

Bjrn Gunnarsson  Bruce D. Marsh  Hugh P. Taylor Jr. 《Journal of Volcanology and Geothermal Research》1998,83(1-2)

The Torfajökull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (225 km³). Within SW-Torfajökull, postglacial mildly alkalic to peralkalic silicic lavas and lava domes (67–74 wt.% SiO₂) have erupted from a family of fissures 1–2.5 km apart within or just outside a large caldera (12×18 km). The silicic lavas show a fissure-dependent variation in composition, and form five chemically distinct units. The lavas are of low crystallinity (0–7 vol.%) and contain phenocrysts in the following order of decreasing abundance: plagioclase (An_10-40), Na-rich anorthoclase (<Or₂₃), clinopyroxene (Fs_37-20), FeTi oxides (Usp_32-60; Ilm_93-88), hornblende (edenitic–ferroedenitic) and olivine (Fo_22-37), with apatite, pyrrhotite and zircon as accessory phases. The phenocryst assemblage (0.2–4.0 mm) consistently exhibits pervasive disequilibrium with the host melt (glass). Xenoliths include sparse, disaggregated, and partially fused leucocratic fragments as well as amphibole-bearing rocks of broadly intermediate composition. The values of the silicic lavas are in the range 3.6–4.4, and these are lower than the values of comagmatic, contemporaneous basaltic extrusives within SW-Torfajökull, implying that the former can not be derived from the latter by simple fractional crystallization. FeTi-oxide geothermometry reveals temperatures as low as 750–800°C. To explain the fissure-dependent chemical variations, depletions, low FeTi-oxide temperatures and pervasive crystal-melt disequilibrium, we propose the extraction and collection of small parcels of silicic melt from originally heterogeneous basaltic crustal rock through heterogeneous melting and wall rock collapse (solidification front instability, SFI). The original compositional heterogeneity of the source rock is due to (1) silicic segregations, in the form of pods and lenses characteristically formed in the upper parts of gabbroic intrusives, and (2) extreme isostatic subsidence of the earlier, less differentiated lavas of the Torfajökull central volcano. Ridge migration into older crustal terranes, coupled with establishment of concentrated volcanism at central volcanoes like Torfajökull due to propagating regional fissure swarms, supplies the heat source for this overall process. Continued magmatism in these fissures promotes extensive prograde heating of older crust and the progressive vitality and rise of the central volcano magmatic system that leads to, respectively, SFI and subsidence melting. The ensuing silicic melts (with relict crystals) are extracted, collected and extruded before reaching complete internal equilibrium. Chemically, this appears as a two-stage process of crystal fractionation. In general, the accumulation of high-temperature basaltic magmas at shallow depths beneath the Icelandic rift zones and major central volcanoes, coupled with unique tectonic conditions, allows large-scale reprocessing and recycling of the low- , hydrothermally altered Icelandic crust. The end result is a compositionally bimodal proto-continental crust.  相似文献   

Thermodynamic data from redox reactions at high temperatures. I. An experimental and theoretical assessment of the electrochemical method using stabilized zirconia electrolytes,with revised values for the Fe-“FeO”, Co-CoO,Ni-NiO and Cu-Cu2O oxygen buffers,and new data for the W-WO2 buffer     

Hugh St. C. O'Neill  Mark I. Pownceby 《Contributions to Mineralogy and Petrology》1993,114(3):296-314

The defined by the reaction 6 MnO+O₂ =2 Mn₃O₄ has been determined from 917 to 1,433 K using electrochemical cells (with calcia-stabilized zirconta, CSZ) of the type: Steady emfs were achieved rapidly at all temperatures on both increasing and decreasing temperature, indicating that the MnO-Mn₃O₄ oxygen buffer equilibrates relatively easily. It therefore makes a useful alternative choice in experimental petrology to Fe₂O₃-Fe₃O₄ for buffering oxygen potentials at oxidized values. The results are (in J/mol, temperature in K, reference pressure 1 bar); (±200)=-563,241+1,761.758T-220.490T inT+0.101819T ² with an uncertainty of ±200 J/mol. Third law analysis of these data, including a correction for the deviations in stoichiometry of MnO, impliesS _298.15 for Mn₃O₄ of 166.6 J/K · mol, which is 2.5 J/K · mol higher than the calorimetric determination of Robie and Hemingway (1985). The low value of the calorimetric entropy may be due to incomplete ordering of the magnetic spins. The third law value of _r H _298.15 ⁰ is-450.09 kJ/mol, which is significantly different from the calorimetric value of-457.5±3.4 kJ/mol, calculated from _f H _298.15 ⁰ of MnO and Mn₃O₄, implying a small error in one or both of these latter.  相似文献   

The role of trace elements in controlling H incorporation in San Carlos olivine     

Peter?M.?E.?TollanEmail authorView author&#;s OrcID profile  Hugh?St.?C.?O’Neill  J?rg?Hermann 《Contributions to Mineralogy and Petrology》2018,173(11):89

We conducted a series of hydroxylation experiments using mm-sized cuboids cut from six different crystals of San Carlos olivine with a range of trace-element concentrations. The cuboids were pre-annealed and then hydroxylated under identical conditions, ensuring that variation in the amounts of H incorporated depended only on the compositional variables. The pre-anneal was at 1400 °C, atmospheric pressure and an oxygen fugacity equivalent to Δlog FMQ?+?1, with the subsequent hydroxylation at 800 °C and 1.5 GPa, for 3 days. Hydrogen was incorporated into all six crystals by the four main substitution mechanisms [Si], [Mg], [Ti] and [triv], with homogeneous H contents in the cores of the crystals, indicating H diffusion rates faster than 10^??11 m²/s. Total H as H₂O in the homogeneous cores calculated by summing all the infrared absorbance bands ranges from 13 to 27 wt. ppm. The total H₂O in the six pre-annealed crystals is poorly correlated with any measured compositional variable. However, when the H₂O associated with individual infrared bands is compared, clear trends emerge. The intensity of absorption bands at 3572 and 3525 cm^??1 are strongly correlated with Ti concentrations, whose range in the six crystals exceeds an order of magnitude. Bands between 3400 and 3300 cm^??1, correlate negatively with Na⁺, but are positively correlated with the difference between molar Cr³⁺ and Na⁺. This highlights a previously unrecognised role for Na in suppressing H incorporation in natural olivines. The results confirm the important role that the trace constituents of olivine play in H incorporation. Two of these trace elements, Na and Ti, tend to be similarly enriched or depleted by partial melting or metasomatism of the mantle, but have opposite effects on H incorporation, with Ti enhancing it but Na suppressing it. Models estimating the effect of H in olivine on mantle rheology must, therefore, consider carefully the availability of these trace elements.  相似文献   

Sediment budget analysis of slope–channel coupling and in-channel sediment storage in an upland catchment, southeastern Australia     

Hugh G. Smith  Deirdre Dragovich 《Geomorphology》2008,101(4):643-654

Slope–channel coupling and in-channel sediment storage can be important factors that influence sediment delivery through catchments. Sediment budgets offer an appropriate means to assess the role of these factors by quantifying the various components in the catchment sediment transfer system. In this study a fine (< 63 µm) sediment budget was developed for a 1.64-km² gullied upland catchment in southeastern Australia. A process-based approach was adopted that involved detailed monitoring of hillslope and bank erosion, channel change, and suspended sediment output in conjunction with USLE-based hillslope erosion estimation and sediment source tracing using ¹³⁷Cs and ²¹⁰Pb_ex. The sediment budget developed from these datasets indicated channel banks accounted for an estimated 80% of total sediment inputs. Valley floor and in-channel sediment storage accounted for 53% of inputs, with the remaining 47% being discharged from the catchment outlet. Estimated hillslope sediment input to channels was low (5.7 t) for the study period compared to channel bank input (41.6 t). However an estimated 56% of eroded hillslope sediment reached channels, suggesting a greater level of coupling between the two subsystems than was apparent from comparison of sediment source inputs. Evidently the interpretation of variability in catchment sediment yield is largely dependent on the dynamics of sediment supply and storage in channels in response to patterns of rainfall and discharge. This was reflected in the sediment delivery ratios (SDR) for individual measurement intervals, which ranged from 1 to 153%. Bank sediment supply during low rainfall periods was reduced but ongoing from subaerial processes delivering sediment to channels, resulting in net accumulation on the channel bed with insufficient flow to transport this material to the catchment outlet. Following the higher flow period in spring of the first year of monitoring, the sediment supplied to channels during this interval was removed as well as an estimated 72% of the sediment accumulated on the channel bed since the start of the study period. Given the seasonal and drought-dependent variability in storage and delivery, the period of monitoring may have an important influence on the overall SDR. On the basis of these findings, this study highlights the potential importance of sediment dynamics in channels for determining contemporary sediment yields from small gullied upland catchments in southeastern Australia.  相似文献   

Measurement of persistent organic pollutants (POPs) in plastic resin pellets from remote islands: toward establishment of background concentrations for International Pellet Watch   总被引：1，自引：0，他引：1  

Heskett M  Takada H  Yamashita R  Yuyama M  Ito M  Geok YB  Ogata Y  Kwan C  Heckhausen A  Taylor H  Powell T  Morishige C  Young D  Patterson H  Robertson B  Bailey E  Mermoz J 《Marine pollution bulletin》2012,64(2):445-448

Plastic resin pellets collected from remote islands in the Pacific, Atlantic, and Indian Oceans and the Caribbean Sea were analyzed for polychlorinated biphenyls (PCBs), dichloro-diphenyltrichloroethane and its degradation products (DDTs), and hexachlorocyclohexanes (HCHs). Concentrations of PCBs (sum of 13 congeners) in the pellets were 0.1-9.9 ng/g-pellet. These were 1-3 orders of magnitude smaller than those observed in pellets from industrialized coastal shores. Concentrations of DDTs in the pellets were 0.8-4.1 ng/g-pellet. HCH concentrations were 0.6-1.7 ng/g-pellet, except for 19.3 ng/g-pellet on St. Helena, where current use of lindane is likely influence. This study provides background levels of POPs (PCBs<10 ng/g-pellet, DDTs <4 ng/g-pellet, HCHs <2 ng/g-pellet) for International Pellet Watch. Sporadic large concentrations of POPs were found in some pellet samples from remote islands and should be considered in future assessments of pollutants on plastic debris.  相似文献   

Using dissolved H2O in rhyolitic glasses to estimate palaeo-ice thickness during a subglacial eruption at Bláhnúkur (Torfaj?kull, Iceland)     

Jacqueline Owen  Hugh Tuffen  David W. McGarvie 《Bulletin of Volcanology》2012,74(6):1355-1378

The last decade has seen the refinement of a technique for reconstructing palaeo-ice thicknesses based on using the retained H₂O and CO₂ content in glassy eruptive deposits to infer quenching pressures and therefore ice thicknesses. The method is here applied to Bláhnúkur, a subglacially erupted rhyolitic edifice in Iceland. A decrease in water content from ～0.7?wt.% at the base to ～0.3?wt.% at the top of the edifice suggests that the ice was 400?m thick at the time of the eruption. As Bláhnúkur rises 350?m above the surrounding terrain, this implies that the eruption occurred entirely within ice, which corroborates evidence obtained from earlier lithofacies studies. This paper presents the largest data set (40 samples) so far obtained for the retained volatile contents of deposits from a subglacial eruption. An important consequence is that it enables subtle but significant variations in water content to become evident. In particular, there are anomalous samples which are either water-rich (up to 1?wt.%) or water-poor (～0.2?wt.%), with the former being interpreted as forming intrusively within hyaloclastite and the latter representing batches of magma that were volatile-poor prior to eruption. The large data set also provides further insights into the strengths and weaknesses of using volatiles to infer palaeo-ice thicknesses and highlights many of the uncertainties involved. By using examples from Bláhnúkur, the quantitative use of this technique is evaluated. However, the relative pressure conditions which have shed light on Bláhnúkur’s eruption mechanisms and syn-eruptive glacier response show that, despite uncertainties in absolute values, the volatile approach can provide useful insight into the mechanisms of subglacial rhyolitic eruptions, which have never been observed.  相似文献   

Thermodynamic data from redox reactions at high temperatures. VI. Thermodynamic properties of CoO–MnO solid solutions from emf measurements     

Mark I. Pownceby  Hugh St.C. O'Neill 《Contributions to Mineralogy and Petrology》2000,140(1):28-39

Activities of CoO in (Co,Mn)O solid solutions in contact with metallic Co have been determined on ten compositions ranging from 0.12 to 0.84 X_CoO in order to calibrate the divariant equilibrium between (Co,Mn)O oxide solutions and Co metal as an oxygen fugacity sensor for application in experimental petrology. Experiments were conducted over the temperature range 900–1300 K at 1 bar, using an electrochemical technique with oxygen-specific calcia-stabilized zirconia (CSZ) electrolytes. Co + CoO or Fe + FeO was used as the reference electrode. Compositions of the (Co,Mn)O solid solutions were measured after each run by electron microprobe, and these were checked for internal consistency by measuring the lattice parameter by X-ray diffraction. Activity–composition relations were fitted to the Redlich–Kister formalism. (Co,Mn)O solid solutions exhibit slight positive deviations from ideality, which are symmetrical (corresponding to a regular solution mixing model) across the entire composition range with A₀ ^G = 3690(±47) Jmol⁻¹. Excess entropies and enthalpies were also derived from the emf data and gave S^ex=0.77(±0.08) JK⁻¹mol⁻¹ and H^ex=4558(±90) Jmol⁻¹ respectively. The experimental data from this study have been used to formulate the (Co,Mn)O/Co oxygen fugacity sensor to give an expression: where μO₂ ^CoCoO=−492,186 + 509.322 T − 53.284 T lnT + 0.02518 T², taken from O'Neill and Pownceby (1993). Received: 10 September 1999 / Accepted: 4 April 2000  相似文献   

Monitoring forest areas from continental to territorial levels using a sample of medium spatial resolution satellite imagery     

Hugh Eva  Silvia Carboni  Frédéric Achard  Nicolas Stach  Laurent Durieux  Jean-François Faure  Danilo Mollicone 《ISPRS Journal of Photogrammetry and Remote Sensing》2010,65(2):191-197

A global systematic sampling scheme has been developed by the UN FAO and the EC TREES project to estimate rates of deforestation at global or continental levels at intervals of 5 to 10 years. This global scheme can be intensified to produce results at the national level. In this paper, using surrogate observations, we compare the deforestation estimates derived from these two levels of sampling intensities (one, the global, for the Brazilian Amazon the other, national, for French Guiana) to estimates derived from the official inventories. We also report the precisions that are achieved due to sampling errors and, in the case of French Guiana, compare such precision with the official inventory precision.We extract nine sample data sets from the official wall-to-wall deforestation map derived from satellite interpretations produced for the Brazilian Amazon for the year 2002 to 2003. This global sampling scheme estimate gives 2.81 million ha of deforestation (mean from nine simulated replicates) with a standard error of 0.10 million ha. This compares with the full population estimate from the wall-to-wall interpretations of 2.73 million ha deforested, which is within one standard error of our sampling test estimate. The relative difference between the mean estimate from sampling approach and the full population estimate is 3.1%, and the standard error represents 4.0% of the full population estimate.This global sampling is then intensified to a territorial level with a case study over French Guiana to estimate deforestation between the years 1990 and 2006. For the historical reference period, 1990, Landsat-5 Thematic Mapper data were used. A coverage of SPOT-HRV imagery at 20 m × 20 m resolution acquired at the Cayenne receiving station in French Guiana was used for year 2006.Our estimates from the intensified global sampling scheme over French Guiana are compared with those produced by the national authority to report on deforestation rates under the Kyoto protocol rules for its overseas department. The latter estimates come from a sample of nearly 17,000 plots analyzed from same spatial imagery acquired between year 1990 and year 2006. This sampling scheme is derived from the traditional forest inventory methods carried out by IFN (Inventaire Forestier National). Our intensified global sampling scheme leads to an estimate of 96,650 ha deforested between 1990 and 2006, which is within the 95% confidence interval of the IFN sampling scheme, which gives an estimate of 91,722 ha, representing a relative difference from the IFN of 5.4%.These results demonstrate that the intensification of the global sampling scheme can provide forest area change estimates close to those achieved by official forest inventories (<6%), with precisions of between 4% and 7%, although we only estimate errors from sampling, not from the use of surrogate data.Such methods could be used by developing countries to demonstrate that they are fulfilling requirements for reducing emissions from deforestation in the framework of an REDD (Reducing Emissions from Deforestation in Developing Countries) mechanism under discussion within the United Nations Framework Convention on Climate Change (UNFCCC). Monitoring systems at national levels in tropical countries can also benefit from pan-tropical and regional observations, to ensure consistency between different national monitoring systems.  相似文献   

Past and present permafrost as an indicator of climate change     

Hugh M. French 《Polar research》1999,18(2):269-274

The permafrost history of the high northern latitudes over the last two million years indicates that perennially frozen ground formed and thawed repeatedly, probably in close synchronicity with the climate changes that led to the expansion and subsequent shrinkage of continental ice sheets. The early stages of the Pleistocene are the least known and the changes that occurred in the Late Pleistocene and early Holocene are the best known.
Evidence that permafrost is degrading in response to the current global warming trend is difficult to ascertain. The clearest signals are probably provided by changes in permafrost distribution in the sub-Arctic regions. at the extreme southern fringes of the discontinuous permafrost zone.  相似文献   

Removal of Perchloroethylene from a Layered Soil System by Steam Flushing   总被引：2，自引：0，他引：2  

Hugh Y. She  Brent E. Sleep 《Ground Water Monitoring & Remediation》1999,19(2):70-77

Steam flushing experiments were concluded in a two-dimensional chamber containing two layers of F75 silica sand separated by a layer of finer F10 silica sand. Perchloroelhylene (PCE), which had spilled into the chamber under water-saturated conditions, formed a pool on the F110 sand layer. Steam was injected above the F110 sand layer. Temperatures, moisture content, and PCE concentrations in the chamber were monitored. Samples, taken from the various locations in the sand chamber, indicated that complete removal of PCE from the steam zone was achieved, with an 84% overall recovery. Some downward displacement of PCE-contaminated water through the F110 sand layer was observed and a small amount of gravity override occurred. Channeling of steam was minimal. The experiment indicates that steam flushing may be used successfully for removal of PCE from relatively homogeneous soils. Issues of gravity override and downward mobilization of contaminants must be considered in applying steam flushing at the field scale.  相似文献