Compositional gradients surrounding spherulites in obsidian and their relationship to spherulite growth and lava cooling     

James E. Gardner  Kenneth S. Befus  James Watkins  Marc Hesse  Nathan Miller 《Bulletin of Volcanology》2012,74(8):1865-1879

Spherical masses of crystal fibers (spherulites) crystalize from rhyolitic melt/glass mainly in response to significant undercooling while lava cools. Spherulite growth should induce compositional gradients in the surrounding glass from expulsion of incompatible constituents and diffusion of those constituents away from the spherulite. Finite-difference numerical modeling of one-dimensional diffusion, in which diffusivities are allowed to vary with temperature, is used to investigate how compositional gradients reflect spherulite growth and lava cooling. Overall, three forms of gradients are identified. Elements that diffuse quickly are expelled from the spherulite but then migrate away too quickly to become enriched at the boundary of the spherulite. Elements that diffuse slowly are trapped within the growing spherulite. Between those endmembers are elements that are not trapped, yet diffuse slow enough that they become enriched at the contact. Their slow diffusion away then elevates their concentrations in the surrounding glass. How enriched those elements are at the spherulite-matrix interface and how far their enrichments extend outwards into the glass reflect how spherulites grow and thermal conditions during growth. Concentrations of H₂O, Rb, F, Li, Cl, Na, K, Sr, Cs, Ba, and Be were measured in and around spherulites in obsidian from a 4.7?±?1?km³ rhyolite lava dome erupted from Tequila volcano, Mexico. Measurable concentration gradients are found for H₂O, Rb, and F. Attributes of those gradients and the behaviors of the other elements are in accord with their experimentally constrained diffusivities. Spherulites appear to have grown following radial, rather than volumetric, growth. The observed gradients (and lack of others) are more consistent with growth mainly below the glass transition, which would necessitate the dome cooling at ca. 10^?5 to 10^?7?°C?s^?1. Such slow cooling is consistent with the relatively large volume of the dome.  相似文献   

Uncoupled viral and bacterial distributions in coral reef waters of Tuamotu Archipelago (French Polynesia)     

Bouvy M  Combe M  Bettarel Y  Dupuy C  Rochelle-Newall E  Charpy L 《Marine pollution bulletin》2012,65(10-12):506-515

This study examined the distribution of virioplankton and bacterioplankton in two coral reef systems (Ahe and Takaroa atolls) in the Tuamotu Archipelago, in comparison with the surrounding oligotrophic ocean. Mean concentrations of 4.8×10(5) and 6.2×10(5)cellsml(-1) for bacteria and 8.1×10(6) and 4.3×10(6)VLP(virus-like particle) ml(-1) were recorded in Ahe and Takaroa lagoons, respectively. Chlorophyll-a concentrations and dissolved organic matter were higher in Ahe whereas (3)H thymidine incorporation rates were higher in Takaroa. First data on lytic and lysogenic strategies of phages in coral reef environments were discussed in this paper. The fraction of visibly infected cells by viruses was negligible regardless of the lagoon station (mean=0.15%). However, the fraction of lysogenic cells ranged between 2.5% and 88.9%. Our results suggest that the distribution patterns of virioplankton are apparently not coupled to the spatial dynamics of the bacterioplankton communities.  相似文献   

Pearl oysters Pinctada margaritifera grazing on natural plankton in Ahe atoll lagoon (Tuamotu archipelago, French Polynesia)   总被引：1，自引：0，他引：1  

Fournier J  Dupuy C  Bouvy M  Couraudon-Réale M  Charpy L  Pouvreau S  Le Moullac G  Le Pennec M  Cochard JC 《Marine pollution bulletin》2012,65(10-12):490-499

In atoll lagoons of French Polynesia, growth and reproduction of pearl oysters are mainly driven by plankton concentration. However, the actual diet of black-lip pearl oysters Pinctada margaritifera in these lagoons is poorly known. To fill this gap, we used the flow through chamber method to measure clearance rates of P. margaritifera in Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia). We found: (i) that pearl oysters cleared plankton at a rate that was positively related to plankton biovolume, (ii) that nanoflagellates were the main source of carbon for the pearl oysters, and (iii) that the quantity and origin of carbon filtrated by pearl oysters was highly dependent on the concentration and composition of plankton. These results provide essential elements for the comprehension of growth and reproduction variability of pearl oysters in atoll lagoons of French Polynesia.  相似文献   

Molecular and isotopic composition of hydrate-bound and dissolved gases in the southern basin of Lake Baikal, based on an improved headspace gas method   总被引：1，自引：1，他引：0  

Hirotoshi Sakagami  Nobuo Takahashi  Akihiro Hachikubo  Hirotsugu Minami  Satoshi Yamashita  Hitoshi Shoji  Oleg Khlystov  Gennadiy Kalmychkov  Mikhail Grachev  Marc De Batist 《Geo-Marine Letters》2012,32(5-6):465-472

Assessments of the molecular and isotopic composition of hydrate-bound and dissolved gases in pore water were conducted during the multi-phase gas hydrate project (MHP-09) cruise VER09-03 to the southern basin of Lake Baikal in September 2009. To avoid changes in gas composition during core sampling and transport, various headspace methods were investigated aimed at preserving the dissolved gases in pore water. When distilled water was added to the sediment samples, the concentrations of carbon dioxide and oxygen decreased because of dissolution into the water and/or microbial consumption. When the headspace was not flushed with inert gases, trace levels of hydrogen and ethylene were detected. The findings suggest that best preparation is achieved by flushing the headspace with helium, and adding a saturated aqueous solution of sodium chloride. This improved headspace method served to examine the molecular and isotopic compositions of gas samples retrieved at several new sites in the southern basin. Methane was the major component, and the proportion of ethane ranged widely from 0.0009 to 1.67?mol% of the total hydrocarbon gases. The proportions of propane and higher hydrocarbons were small or less than their detection limits. The carbon isotope signatures suggest that microbial-sourced methane and ethane were dominant in the Peschanka study area, whereas ethane was of thermogenic origin at all other study sites in the southern basin of Lake Baikal.  相似文献   

Rheomorphic ignimbrites of the Rogerson Formation,central Snake River plain,USA: record of mid-Miocene rhyolitic explosive eruptions and associated crustal subsidence along the Yellowstone hotspot track     

Thomas R. Knott  Marc K. Reichow  Michael J. Branney  David R. Finn  Robert S. Coe  Michael Storey  Bill Bonnichsen 《Bulletin of Volcanology》2016,78(4):23

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A study of dissipation of wind-waves by mud at Cassino Beach,Brazil: Prediction and inversion     

W. Erick Rogers  K. Todd Holland 《Continental Shelf Research》2009

The impact of a non-rigid seafloor on the wave climate at Cassino Beach, Brazil, May–June 2005 is studied using field measurements and a numerical wave model. The measurements consist of wave data at four locations; rheology and mud thickness from grab samples; and an estimate of the horizontal distribution of mud based on echo-soundings. The dissipation of waves by a non-rigid bottom is represented in the wave model by treating the mud layer as a viscous fluid. Applied for 431 time periods, the model without this type of dissipation has a strong tendency to overpredict nearshore wave energy, except during a period of large storm waves. Two model variations which include this dissipation have a modest tendency to underpredict the nearshore wave energy. An inversion methodology is developed and applied to infer an alternate mud distribution which, when used with the wave model, yields the observed waveheights.  相似文献   

On the dynamics of mud deposits in coastal areas     

Susana B. Vinzon  Lauro J. CalliariK. Todd. Holland  Johan C. Winterwerp 《Continental Shelf Research》2009

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Reply to comment on ‘Low‐frequency microtremor anomalies at an oil and gas field in Voitsdorf,Austria’ by Marc‐André Lambert,Stefan M. Schmalholz,Erik H. Saenger and Brian Steiner,Geophysical Prospecting 57, 393–411     

Marc‐André Lambert  Stefan M. Schmalholz  Erik H. Saenger  Brian Steiner 《Geophysical Prospecting》2010,58(2):341-346

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The granulitic impactite suite: Impact melts and metamorphic breccias of the early lunar crust     

Janet A. USHING  G. Jeffrey TAYLOR  Marc D. NORMAN  Klaus KEIL 《Meteoritics & planetary science》1999,34(2):185-195

Abstract— An important and poorly understood group of rocks found in the ancient lunar highlands is called “feldspathic granulitic impactites.” Rocks of the granulite suite occur at most of the Apollo highlands sites as hand samples, rake samples, clasts in breccias, and soil fragments. Most lunar granulites contain 70–80% modal plagioclase, but they can range from anorthosite to troctolite and norite. Previous studies have led to different interpretations for the thermal history of these rocks, including formation as igneous plutons, long-duration metamorphism at high temperatures, and short-duration metamorphism at low temperatures. This paper reports on a study of 24 polished thin sections of lunar granulites from the Apollo 15, 16, and 17 missions. We identify three different textural types of granulitic breccias: poikilitic, granoblastic, and poikilitic-granoblastic breccias. These breccias have similar equilibration temperatures (1100 ± 50 °C), as well as common compositions. Crystal size distributions in two granoblastic breccias reveal that Ostwald ripening took place during metamorphism. Solid-state grain growth and diffusion calculations indicate relatively rapid cooling during metamorphism (0.5 to 50 °C/year), and thermal modeling shows that they cooled at relatively shallow depths (<200 m). In contrast, we conclude that the poikilitic rocks formed by impact melting, whereas the poikilitic-granoblastic rocks were metamorphosed and may have partially melted. These results indicate formation of lunar granulites in relatively small craters (30–90 km in diameter), physically associated with the impact-melt breccia pile, and possibly from fine-grained fragmental precursor lithologies.  相似文献   

Exploring the Solution Space of Semi-structured Geographical Problems Using Genetic Algorithms   总被引：1，自引：0，他引：1  

David A. Bennett  Greg A. Wade  & Marc P. Armstrong 《Transactions in GIS》1999,3(1):51-71

Semi-structured geographical problems are often addressed by groups of decision-makers. Each group member is likely to have a specific set of objectives that they wish to address and a unique perspective on the way in which the problem should be solved. The solution to such problems often requires consensus building and compromise among decision-makers as they attempt to optimize their own criteria. The set of criteria adopted by a particular decision-maker constrains the set of solutions he/she will deem acceptable. Compromise among multiple decision-makers can occur at the intersection of these constrained solution sets. Knowledge about the criteria space, the solution space, and the relation between the two is often incomplete for semi-structured problems. New tools are needed to explore, analyze, and visualize the solution space of a problem with respect to multiple analytical models and criteria. In this research we explore the utility of genetic algorithms as an effective means to: (1) search the solution space of geographical problems; (2) visualize the spatial ramifications of alternative criteria spaces; and (3) identify compromise solutions.  相似文献