Textural Maturity of Cumulates: a Record of Chamber Filling, Liquidus Assemblage, Cooling Rate and Large-scale Convection in Mafic Layered Intrusions   总被引：1，自引：0，他引：1  

Holness  Marian B.; Nielsen  Troels F. D.; Tegner  Christian 《Journal of Petrology》2007,48(1):141-157

Textural maturity describes the extent to which a rock has evolvedfrom the initial reaction-controlled texture towards texturalequilibrium controlled by the minimization of interfacial energy.Solidification in a magma chamber results in the formation ofan impingement texture by the random juxtaposition of planar-sidedgrains. Orthocumulates, in which the initial melt-filled poresare pseudomorphed by later-crystallizing phases, have an ophiticor intersertal texture immediately after complete solidification,which then evolves towards solid-state equilibrium by roundingof initially planar grain boundaries and an increase in themedian dihedral angle subtended at the junctions of two primocrysticgrains with the interstitial phase. The bulk of the increasein angle occurs just below the solidus temperature in kilometre-scalemafic plutons. Quantification of textural maturity via measurementof dihedral angle populations in troctolitic and gabbroic cumulatesfrom the Rum Eastern Layered Intrusion and the Skaergaard Intrusiondemonstrates that the rocks preserve a record of thermal eventsrelated to magma chamber replenishment and the onset of chamber-wideconvection. Textural maturity is also a function of the liquidusphase assemblage: for systems in which only olivine and plagioclaseare liquidus (i.e. cumulus) phases in the main magma body abovethe crystal mush, the texture is significantly less mature thanthat in systems in which clinopyroxene is an additional liquidusphase. The difference in textural maturity reflects differencesin the cooling and solidification rate, and demonstrates directlythat the liquidus phase assemblage plays a role in determiningthe thermal history of plutons. KEY WORDS: cumulates; dihedral angles; Rum; Skaergaard; textures  相似文献   

Large thallium isotopic variations in iron meteorites and evidence for lead-205 in the early solar system     

Sune G. Nielsen  Mark Rehkämper  Alex N. Halliday 《Geochimica et cosmochimica acta》2006,70(10):2643-2657

Lead-205 decays to ²⁰⁵Tl with a half-life of 15 Myr and should have been present in the early solar system according to astrophysical models. However, despite numerous attempts, Tl isotopic measurements of meteorites have been unable to demonstrate convincingly its former presence. Here, we report large (∼5‰) variations in Tl isotope composition in metal and troilite fragments from a range of iron meteorites that were determined at high precision using multiple collector inductively coupled plasma mass spectrometry. The Tl isotopic compositions of seven metal samples of the IAB iron meteorites Toluca and Canyon Diablo define a correlation with ²⁰⁴Pb/²⁰³Tl. When interpreted as an isochron, this corresponds to an initial ²⁰⁵Pb/²⁰⁴Pb ratio of (7.4 ± 1.0) × 10⁻⁵. Alternative explanations for the correlation, such as mixing of variably mass-fractionated meteorite components or terrestrial contamination are harder to reconcile with independent constraints. However, troilite nodules from Toluca and Canyon Diablo contain Tl that is significantly less radiogenic than co-existing metal with isotope compositions that are variable and decoupled from ²⁰⁴Pb/²⁰³Tl. These effects are similar to those recently reported by others for Fe and Ni isotopes in iron meteorite sulfides and appear to be the result of kinetic stable isotope fractionation during diffusion. Though it cannot conclusively be shown that the metal fragments are unaffected by the secondary processes that disturbed the troilites, mass balance modeling indicates that the alteration of the troilites is unlikely to have significantly affected the Tl isotope compositions of the co-existing metals. It is therefore reasonable to conclude that the IAB metal isochron is a product of the in situ decay of ²⁰⁵Pb. If the I-Xe ages of IAB silicate inclusions record the same event as the ²⁰⁵Pb-²⁰⁵Tl chronometer then crystallization of the IAB metal was probably completed between 10 and 20 Myr after the condensation of the first solids. This implies an initial solar system ²⁰⁵Pb/²⁰⁴Pb of (1.0-2.1) × 10⁻⁴, which is in excellent agreement with recently published astrophysical predictions. Similar calculations yield an initial solar system Tl isotope composition of ε²⁰⁵Tl = −2.8 ± 1.7. The Tl isotopic composition and concentration of the silicate Earth depends critically on the timing and mechanism of core formation and Earth’s volatile element depletion history. Modeling of the Earth’s accretion and core formation using the calculated initial solar system Tl isotope composition and ²⁰⁵Pb/²⁰⁴Pb, however, does not yield reasonable results for the silicate Earth unless either the Earth lost Tl and Pb late in its accretion history or the core contains much higher concentrations of Pb and Tl than are found in iron meteorites.  相似文献   

Fluid flow and methane occurrences in the Disko Bugt area offshore West Greenland: indications for gas hydrates?     

Tove Nielsen  Troels Laier  Antoon Kuijpers  Tine L. Rasmussen  Naja E. Mikkelsen  Niels Nørgård-Pedersen 《Geo-Marine Letters》2014,34(6):511-523

The present study is the first to directly address the issue of gas hydrates offshore West Greenland, where numerous occurrences of shallow hydrocarbons have been documented in the vicinity of Disko Bugt (Bay). Furthermore, decomposing gas hydrate has been implied to explain seabed features in this climate-sensitive area. The study is based on archive data and new (2011, 2012) shallow seismic and sediment core data. Archive seismic records crossing an elongated depression (20×35 km large, 575 m deep) on the inner shelf west of Disko Bugt (Bay) show a bottom simulating reflector (BSR) within faulted Mesozoic strata, consistent with the occurrence of gas hydrates. Moreover, the more recently acquired shallow seismic data reveal gas/fluid-related features in the overlying sediments, and geochemical data point to methane migration from a deeper-lying petroleum system. By contrast, hydrocarbon signatures within faulted Mesozoic strata below the strait known as the Vaigat can be inferred on archive seismics, but no BSR was visible. New seismic data provide evidence of various gas/fluid-releasing features in the overlying sediments. Flares were detected by the echo-sounder in July 2012, and cores contained ikaite and showed gas-releasing cracks and bubbles, all pointing to ongoing methane seepage in the strait. Observed seabed mounds also sustain gas seepages. For areas where crystalline bedrock is covered only by Pleistocene–Holocene deposits, methane was found only in the Egedesminde Dyb (Trough). There was a strong increase in methane concentration with depth, but no free gas. This is likely due to the formation of gas hydrate and the limited thickness of the sediment infill. Seabed depressions off Ilulissat Isfjord (Icefjord) previously inferred to express ongoing gas release from decomposing gas hydrate show no evidence of gas seepage, and are more likely a result of neo-tectonism.  相似文献   

Development of python-FALL3D: a modified procedure for modelling volcanic ash dispersal in the Asia-Pacific region     

A. N. Bear-Crozier  Nugraha Kartadinata  Anjar Heriwaseso  Ole Nielsen 《Natural Hazards》2012,64(1):821-838

Volcanic ash is the most widespread of all volcanic hazards and has the potential to affect hundreds of thousands, or even millions, of people in the densely populated islands of Indonesia. There is limited information available for this region on the hazard posed by volcanic ash, particularly from volcanoes that have not erupted in recent times. There is a need for computational models capable of accurately predicting volcanic ash dispersal at ground level when coupled with field observations of historical or ongoing eruptive activity. To maximise the effectiveness of such models, they should be readily accessible, easy to use and well tested. Geoscience Australia in collaboration with the Australia-Indonesia Facility for Disaster Reduction and the Indonesian Centre for Volcanology and Geohazard Mitigation has collaboratively adapted an existing open-source volcanic ash dispersion model for use in Indonesia. The core model is the widely used, open-source volcanic ash dispersion model FALL3D. A Python wrapper (name here python-FALL3D) has been developed, which modifies the modelling procedure of FALL3D in order to simplify its use for those with little or no background in computational modelling. The modified procedure does not alter the core functionality of FALL3D, but simplifies the modelling procedure by streamlining the installation process, automating both the pre-processing of input meteorological datasets and configuring and executing each utility program in a single-step process. An application example was presented using python-FALL3D for an active volcano in West Java, Indonesia. The example showed that communities located on the western side of Gunung Gede are always susceptible to volcanic ash ground loading regardless of the seasonal variations in wind conditions, whereas communities on the eastern side of Gunung Gede have a marked increase in susceptibility to ground loading during rainy season conditions when prevailing winds include a strong easterly component.  相似文献   

A Temporal Variant‐Invariant Validation Approach for Agent‐based Models of Landscape Dynamics     

Christopher Bone  Bart Johnson  Max Nielsen‐Pincus  Eric Sproles  John Bolte 《Transactions in GIS》2014,18(2):161-182

Agent‐based modeling provides a means for addressing the way human and natural systems interact to change landscapes over time. Until recently, evaluation of simulation models has focused on map comparison techniques that evaluate the degree to which predictions match real‐world observations. However, methods that change the focus of evaluation from patterns to processes have begun to surface; that is, rather than asking if a model simulates a correct pattern, models are evaluated on their ability to simulate a process of interest. We build on an existing agent‐based modeling validation method in order to present a temporal variant‐invariant analysis (TVIA). The enhanced method, which focuses on analyzing the uncertainty in simulation results, examines the degree to which outcomes from multiple model runs match some reference to how land use parcels make the transition from one land use class to another over time. We apply TVIA to results from an agent‐based model that simulates the relationships between landowner decisions and wildfire risk in the wildland‐urban interface of the southern Willamette Valley, Oregon, USA. The TVIA approach demonstrates a novel ability to examine uncertainty across time to provide an understanding of how the model emulates the system of interest.  相似文献   

Observations of ionospheric flows and particle precipitation following a Sudden Commencement     

E. Nielsen  F. Honary 《Annales Geophysicae》2000,18(8):908-917

On May 4, 1998, at 0227 UT an interplanetary shock crossed the WIND spacecraft, and half an hour later a Sudden Commencement occurred. Coinciding with the Sudden Commencement a rapid intensification of the flux of particle precipitation into the ionosphere was observed. Evidence is presented that the ionospheric electric fields were influenced by the associated dynamic variations of the ionospheric conductivities. Following the initial phase the ionospheric flow speeds increased rapidly over the next 20 min to more than 2000 m/s, in agreement with an increased effective coupling of the solar wind energy to the magnetosphere following the interplanetary shock that caused the Sudden Commencement. These strong flows were meandering in latitude, a type of plasma flow modulation that has been reported before to occur during Omega band events: a string of alternating field-aligned currents propagating eastward. The riometer absorption was found to be at a minimum in regions associated with outward directed field aligned currents. The riometer absorption regions (the regions of particle precipitation) were drifting with E × B drift speed of the ionospheric electrons.  相似文献   

Automated rainfall simulator for variable rainfall on urban green areas   总被引：1，自引：0，他引：1  

Kristoffer T. Nielsen  Per Moldrup  Sren Thorndahl  Jesper E. Nielsen  Lene B. Duus  Sren H. Rasmussen  Mads Uggerby  Michael R. Rasmussen 《水文研究》2019,33(26):3364-3377

Rainfall simulators can enhance our understanding of the hydrologic processes affecting the total runoff to urban drainage systems. This knowledge can be used to improve urban drainage designs. In this study, a rainfall simulator is developed to simulate rainfall on urban green surfaces. The rainfall simulator is controlled by a microcomputer programmed to replicate the temporal variations in rainfall intensity of both historical and synthetic rainfall events with constant rainfall intensity on an area of 1 m². The performance of the rainfall simulator is tested under laboratory conditions with regard to spatial uniformity of the rainfall, the kinetic energy of the raindrops, and the ability to replicate historical and synthetic rainfall events with temporally varying intensity. The rainfall simulator is applied in the field to evaluate its functionality under field conditions and the influence of wind on simulated rainfall. Finally, a field study is carried out on the relationship between runoff, soil volumetric water content, and surface slope. Performance and field tests show that the simulated rainfall has a uniform spatial distribution, whereas the kinetic energy of the raindrops is slightly higher than that of other comparable rainfall simulators. The rainfall simulator performs best in low wind speed conditions. The simulator performs well in replicating historical and synthetic rainfall events by matching both intensity variations and accumulated rainfall depth. The field study shows good correlation between rainfall, runoff, infiltration, soil water content, and surface slope.  相似文献