Water-Saturated and -Undersaturated Melting of Metaluminous and Peraluminous Crustal Compositions at 10 kb: Evidence for the Origin of Silicic Magmas in the Taupo Volcanic Zone, New Zealand, and Other Occurrences

The melting relations of two proposed crustal source compositionsfor rhyolitic magmas of the Taupo Volcanic Zone (TVZ), New Zealand,have been studied in a piston-cylinder apparatus at 10 kb totalpressure and a range of water activities generated by H₂O-CO₂vapour. Starting materials were glasses of intermediate composition(65 wt.% Si0₂ representing a metaluminous ‘I-type’dacite and a peraluminous ‘S-type’ greywacke. Crystallizationexperiments were carried out over the temperature range 675to 975?C, with aH₂O values of approximately 1?0, 0?75, 0?5,and 0?25. Talc-pyrex furnace assemblies imposed oxygen fugacitiesclose to quartz-fayalite-magnetite buffer conditions. Assemblages in both compositions remain saturated with quartzand plagioclase through 675–700?C at high aH₂O, 725–750?Cat aH₂O0?5, and 800–875?C at aH₂O0?25, corresponding to<60–70% melting. Concentrations of refractory mineralcomponents (Fe, Mg, Mn, P, Ti) in liquids increase throughoutthis melting interval with increasing temperature and decreasingaH₂O. Biotite and hornblende are the only mafic phases presentnear the solidus in the dacite, compared with biotite, garnet,gedritic orthoamphibole, and tschermakitic clinoamphibole inthe greywacke. Near-solidus melting reactions are of the type:biotite + quartz + plagioclase = amphibole ? garnet, potentiallyreleasing H₂O for dehydration melting in the greywacke, butproducing larger amounts of hornblende and releasing littleH₂O in the dacite. At aH₂O0?25 and temperatures 825–850?C,amphibole dehydration produces anhydrous mineral phases typicalof granulite fades assemblages (clinopyroxene, orthopyroxene,plagioclase?quartz in the dacite; garnet, orthopyroxene, plagioclase?quartzin the greywacke) coexisting with melt proportions as low as40%. Hornblendce-saturated liquids in the dacite are weaklyperaluminous (0?3–1?6 wt.% normative C—within therange of peraluminous TVZ rhyolites), whereas, at aH₂O0?25 andtemperatures 925?C, metaluminous partial melt compositions (upto 1?8 wt.% normative Di) coexist with plagioclase, orthopyroxene,and clinopyroxene. At all water activities, partial melts ofthe greywacke are uniformly more peraluminous (1?5–2?6wt.% normative C), reflecting their saturation in the componentsof more aluminous mafic minerals, particularly garnet and Al-richorthopyroxene. A metaluminous source for the predominantly Di-normativeTVZ rhyolites is therefore indicated. With decreasing aH₂O the stability fields of plagioclase andquartz expand, whereas that of biotite contracts. These changesare reflected in the proportions of normative salic componentsin partial melts of both the dacite and greywacke. At high aH₂O,partial melts are rich in An and Ab and poor in Or (trondhjemitic-tonalitic);with decreasing aH₂O they become notably poorer in An and richerin Or (granodioritic-granitic). These systematic variationsin salic components observed in experimental metaluminous tostrongly peraluminous melts demonstrate that a wide varietyof granitoid magmas may be produced from similar source rocksdepending upon P-T-aH₂O conditions attending partial melting.Some peraluminous granitoids, notably trondhjemitic leucosomesin migmatites, and sodic granodiorites and granites emplacedat deep crustal levels, have bulk compositions similar to nearsolidus melt compositions in both the dacite and greywacke,indicating possible derivation by anatexis without the involvementof a significant restite component.     

北极海洋沉积物中持久性有机污染物分布特征及分子地层学记录的研究

持久性有机污染物(POPs)普遍存在于各种环境介质中,其中有机氯农药(OCPs)和多氯联苯(PCBs)化学性质稳定,在环境中能持久残留.它们经食物链浓缩并在生物体内累积,大多数具有致癌、致畸、致突变效应,构成对人类和生态系统的潜在危害,成为世界公认的全球性环境公害.     

Evolution of a Holocene delta driven by episodic sediment delivery and coseismic deformation, Puget Sound, Washington, USA

Episodic, large‐volume pulses of volcaniclastic sediment and coseismic subsidence of the coast have influenced the development of a late Holocene delta at southern Puget Sound. Multibeam bathymetry, ground‐penetrating radar (GPR) and vibracores were used to investigate the morphologic and stratigraphic evolution of the Nisqually River delta. Two fluvial–deltaic facies are recognized on the basis of GPR data and sedimentary characteristics in cores, which suggest partial emplacement from sediment‐rich floods that originated on Mount Rainier. Facies S consists of stacked, sheet‐like deposits of andesitic sand up to 4 m thick that are continuous across the entire width of the delta. Flat‐lying, highly reflective surfaces separate the sand sheets and comprise important facies boundaries. Beds of massive, pumice‐ and charcoal‐rich sand overlie one of the buried surfaces. Organic‐rich material from that surface, beneath the massive sand, yielded a radiocarbon age that is time‐correlative with a series of known eruptive events that generated lahars in the upper Nisqually River valley. Facies CF consists of linear sandbodies or palaeochannels incised into facies S on the lower delta plain. Radiocarbon ages of wood fragments in the sandy channel‐fill deposits also correlate in time to lahar deposits in upstream areas. Intrusive, sand‐filled dikes and sills indicate liquefaction caused by post‐depositional ground shaking related to earthquakes. Continued progradation of the delta into Puget Sound is currently balanced by tidal‐current reworking, which redistributes sediment into large fields of ebb‐ and flood‐oriented bedforms.     

ANOMALOUS GROUND WARMING VERSUS SURFACE AIR WARMING IN THE CANADIAN PRAIRIE PROVINCES

Modelling results of precision temperature logs made to depths of up to several hundred meters in numerous wells in the Canadian Prairie provinces (mostly Alberta) show evidence of average warming at the ground surface (GST) of 2.1 K (standard deviation = 0.9 K) mostly in the second half of this century. The GST warming signal for which higher frequency noise is cut off by the earth, which acts as a low-pass filter, correlates with the surface air warming (SAT) measured at screen level. A spatial comparison is made between the SAT warming and the GST warming for the last four decades in this region. A GIS (Geographic Information System) area cross tabulation was performed through the intersection of the classes of the ground and surface warming maps with a resulting contingency coefficient C= 0.805. Identical grid samples extracted from the ground warming and surface warming maps were related statistically with a resulting correlation coefficient of r= 0.75. Differences in the magnitudes of the warming exist due to the limited number of compatible data sets, errors in ground warming and air warming reconstructions, and land surface changes affecting the energy balance and subsurface heat fluxes. The influence of these effects requires further study. It is unlikely that all of the sites for which GST warming has been proven to correlate with air warming would have identical topography, vegetation, and hydrogeological disturbances for an area as large as the one under study (about 720,000 km²). The warming effect in the study area, as preserved by the ground, is mainly climate related.     

Monitoring grasshopper and locust habitats in Sahelian Africa using GIS and remote sensing technology†

Abstract

Development programmes in Sahelian Africa are beginning to use geographic information system (GIS) technology. One of the GIS and remote sensing programmes introduced to the region in the late 1980s was the use of seasonal vegetation maps made from satellite data to support grasshopper and locust control. Following serious outbreaks of these pests in 1987, the programme addressed a critical need, by national and international crop protection organizations, to monitor site-specific dynamic vegetation conditions associated with grasshopper and locust breeding. The primary products used in assessing vegetation conditions were vegetation index (greenness) image maps derived from National Oceanic and Atmospheric Administration satellite imagery. Vegetation index data were integrated in a GIS with digital cartographic data of individual Sahelian countries. These near-real-time image maps were used regularly in 10 countries for locating potential grasshopper and locust habitats. The programme to monitor vegetation conditions is currently being institutionalized in the Sahel.     

Generation of equatorial Atlantic warm and cold events in a coupled general circulation model simulation

Warm and cold events in the Gulf of Guinea, characteristic of the tropical interannual variability, can be generated in several ways. This emerges from a statistical analysis of 200 years of interannual variability simulated by a coupled ocean–atmosphere General Circulation Model. The application of a clustering technique to the anomalies of the thermal energy stored in the upper oceanic layers leads to the separation of the events, either warm or cold, into a number of classes, each of them distinguished by a particular generation scheme. The physical mechanisms involved are identified by examining the contributions of the various terms in the mixed layer tendency equation.
Basically, those few classes can be sorted into two larger groups. In one of them, the onset stage is characterized by an eastward propagation of the anomalies. Atmospheric flows play a leading part in the generation of events within this group. In the second group, thermal energy anomalies are generated in situ in the Gulf of Guinea, and it is mixing that gives the most important contribution to the development of the events.
The different classes are related to different seasonal signatures and also to differences in the influence of the El Niño-Southern Oscillation (ENSO). While some of the classes are strongly influenced by ENSO, while in others this influence is not significant. This can explain the barely significant correlations between ENSO and the Atlantic warm events, a feature that the simulation analyzed here shares with the observations.