排序方式： 共有13条查询结果，搜索用时 140 毫秒
1.
Magmatic conditi ons inferred from the composition of the FishCanyon Tuff by Whitney &Stormer (J. Petrology, 26, 726–62(1985)) and Stormer & Whitney (Am. Miner. 70, 52–64(1985)) differ from studies of other ash-flow tuff and calderasystems, in that they consider the magma to have been unzonedand to have resided at {small tilde}9 kb pressure just priorto eruption. We find these conclusions unconvincing becauseof incomplete sampling of the Fish Canyon Tuff, alteration ofthe tuff after emplacement, and, in particular, serious limitationsof coupled Fe-Ti oxide and two-feldspar geothermobarometry. 相似文献
2.
3.
CHRIS A. McLINDEN D. J. CHARTRAND E. GRIFFIOEN J. C. McCONNELL C. T. McELROY 《Journal of Atmospheric Chemistry》1997,26(1):29-64
We have developed models of physically-based cloud and ocean surfacesfor use in photochemical models. These surface models are described in termsof a flux albedo and a normalized reflection function.Through these, the dependence of albedo on wavelength, solar zenithangle, cloud optical depth (cloud surfaces) and surface windspeed (ocean surfaces) are allowed for. In addition, the non-Lambertian nature of these surfaces is accounted for.We have integrated these surfacemodels into a multiple scattering radiative transfer model to assess their effects on the stratospheric radiation field and J-values. This was accomplished by comparison with results obtainedusing Lambertian, constant albedo surfaces. Comparisons of stratospheric radiation fields revealed that boththe wavelength and directional dependences of the cloud and oceansurfaces could be large effects.Differences between calculated J-values varied from 0 to 12% depending upon species, solar zenith angle, andheight.The J-values were then used as input for a chemical box model to examine the effects these surfaces had on stratospheric chemistry. Comparisons were made against box model runs using J-values fromconstant surfaces. Overall, the effect was on the order of 10%.Differences in number densities using these different surfacesvaried with latitude, height and species.Runs were made with and without heterogeneous chemistry. 相似文献
4.
5.
6.
7.
8.
Physical and Chemical Models of Zoned Silicic Magmas: The Loma Seca Tuff and Calabozos Caldera, Southern Andes 总被引：1，自引：3，他引：1
The late Pleistocene Calabozos ash-flow and caldera complexlies in central Chile in a section of the Andean cordillerathat is transitional between dominantly andesitic-to-rhyoliticvolcanism to the north and mafic andesitic and high-aluminabasaltic volcanism to the south. The Calabozos rocks range incomposition from basaltic andesite to rhyodacite and definea high-K calcalkalic suite. They contain 2–25% phenocrystsof plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides,and apatite, ? minor biotite or amphibole. More than 1000km3 of rhyodacitic to dacitic magma erupted atthe Calabozos caldera complex as three major compositionallyzoned ash-flow sheets, Unit L (0?8 Ma), Unit V (0? 30Ma), andUnit S (0?15 Ma) of the Loma Seca Tuff. Phenocryst modes, trace-elementcontents, inferred magmatic volatile contents, and oxygen fugacitiesvary systematically with major-element composition in the tuffs.In the cases of Units V and S, it is possible to reconstructcompositional, thermal, and volatile gradients that existedin density-stratified magma chambers shortly prior to theireruption. The magma graded from crystal-poor, water-rich, andbiotite-bearing rhyodacite in the upper reaches of the chamberto more crystal-rich, water-poor, and amphibole-bearing daciteat deeper levels. Fe-Ti oxide equilibration temperatures are800 to 900?C for rhyodacite and 900 to 950?C for dacite. Magmathat erupted as Unit S was slightly hotter and more oxidizedthan magma that gave rise to Unit V. More mafic magmas wereassociated with the voluminous rhyodacitic to dacitic magmareservoir, as indicated by the presence of andesite and basalticandesite lava flows and by scoria inclusions in Unit V. The compositional suite from basaltic andesite to rhyodacitecan be simulated satisfactorily by crystal-fractionation calculationsbased on major-element phenocryst and rock compositions, andis consistent with modes of the Calabozos rocks. Rhyodacitesof Units V and S, however, are enriched in elements such asRb, Ba, and Zr relative to trace-element contents predictedby crystal-fractionation models. The enrichment can be achievedby assimilation of wall rock or a partial melt of the wall rock.The latter requires that the ratio of assimilation rates tocrystallization rates be between 0?1 and 0?3. Rates of assimilationversus crystallization were greater for Unit S than for UnitV, which is consistent with the lower Fe-Ti oxide temperaturesand less oxidized state of the latter. The Loma Seca Tuff is similar in bulk composition to sanidine-bearingash-flow sheets erupted on ‘mature’ continentalcrust, but it is mineralogically akin to ash-flow tuffs eruptedon ‘immature’ crust. The difference is attributed,in part, to the effect of the density of the crust on the rateof magma ascent at shallow levels. The ascent of large bodiesof silicic magma is slower in silicic (less dense) crust thanin mafic crust, causing the magmas to be erupted at a laterpoint in the crystallization history. 相似文献
9.
The geomorphology of Heard Island-McDonald Island is primarily the product of close interplay between volcanism, glaciation, and vigorous marine processes in a stormy sub-Antarctic environment. The dominant landform is the strato-volcano Big Ben (2745m), which is the highest mountain on Australian territory outside Antarctica. Other volcanic landforms include scoria cones, domes, open vertical volcanic conduits, lava flows and lava tubes. Volcanic activity is ongoing from the summit of Big Ben, and from Samarang Hill on McDonald Island. Early, but unproven, glacial sediments may exist within the Late Miocene - Early Pliocene Drygalski Formation, which forms a 300m high plateau along the northern coast of Heard Island. Growth of the present glaciers, some of which reach sea level, has been a response to progressive growth of the volcanoes. A variety of erosional and depositional glacial landforms is present, including major lateral moraines and extensive hummocky moraines. Vigorous longshore drift and an abundant sediment supply have produced a large spit at the downdrift end of the island, and formed bars from reworked glacigenic sediment that now impound proglacial estuarine lagoons, some of which have grown rapidly over recent decades as tidewater glaciers have retreated. Integrated study of the volcanic, glacial and coastal sequences offers the possibility of constructing a well-dated record of climate change. Research into the geomorphology, surficial sediments, and contemporary geomorphological processes, including glaciofluvial sediment flux, is also important as an aid to environmental management on land, and to management of the adjacent marine environment. 相似文献
10.